The following points highlight the top nine types of adaptation in animals. The types are: 1. Cursorial Adaptation 2. Fossorial Adaptation 3. Scansorial Adaptation 4. Desert Adaptation 5. Volant Adaptation 6. Aquatic Adaptation 7. Adaptations in Cave-Dwellers 8. Deep-Sea Adaptation 9. Parasites and Parasitic Adaptations.
Type # 1. Cursorial Adaptation:
Cursorial adaptation signifies the modifications for attaining speed on hard surface of the earth. In cursorial animals, speed is the prime requisite.
(a) Modifications for Attaining Speed:
Adaptations for attaining speed have been manifested in a variety of ways. The main aim is to offer least resistance in the attainment of speed.
Modifications of contour of the body.The animals are devoid of extra projections which may offer least resistance in attaining speed in the medium through which they move. The- body of cursorial animals is streamlined and spindle-shaped. The body of horse portrays the same modifications.
Modifications of locomotor organs. In cursorial adapted forms the limbs are the main propelling organs, which show great modifications for speed. Of the limbs, the hind- limbs show greater modifications than the forelimbs. This fact is amply recorded in the phylogenetic history of the horse.
The ancestral horse, Eohippus of Eocene period had four digits in the forelimbs and the hind-limbs had only three digits. The forelimbs are modified towards getting the food.
Attainment of digitigrade condition. Primarily the terrestrial forms were plantigrade because they used palm or sole for locomotion. But in cursorial animals the digitigrade condition becomes more perfected. The speediest representatives are digitigrade in nature.
Highest perfection is reached in ungulates where special sole-pads in the form of hoofs are developed at the digital tips to absorb shock and to reduce mechanical friction during fast movement on the hard surface. The ungulates show all the stages in the conversion of the plantigrade condition to the digitigrade condition.
Elongation of the limbs The phenomenon of lengthening of the limbs seems to be the important step towards the attainment of speed. The long bones are conservative, but the digits together with the carpals or tarsals become very much elongated.
The bones of the sole and palm become fused together to form the ‘cannon bone’ which increases the length of the limbs. Such fusion of small bones reduces the friction between small bony pieces during locomotion. The lengthening of the limbs is thus caused by the growth of the distal sectors of the limbs only. The hind-limbs of birds also show speed adaptation and show the fusion of metatarsals.
Reduction of number of digits. The presence of five digits is the usual occurrence in plantigrade animals. But with the attainment of digitigrade condition, there is always the reduction in the number of digits. The climax is observed in horses where the limbs have but one digit.
Reduction of ulna and fibula. There is always a tendency towards the reduction of ulna and fibula. In horse the fibula is reduced to a small vestige. Restriction of movement in one plane. There is always a restriction of movement at one plane excepting the shoulder and the hip.
The articulation of bones prevents universal movement but permits movement in one plane only. The main aim of such adaptation is to increase the locomotory power.
Attainment of bipedality. Attainment of bipedality is the other attribute of cursorial adaptation. Mode of progression by two feet is repeated in many vertebrates. The human feet become secondarily readapted for bipedal movement. Better development of hind-limbs and the reduction of the forelimbs are noted. In bipedal forms the hind-limbs are exclusively the locomotory agent and become cursorially adapted.
The forelimbs become reduced to a considerable extent. Due to disparity of function, the limbs become unequally developed. The forelimbs in cursorial animals, show reduction and usually serve as the prehensive organs.
Maintenance of balance. In bipedal animals maintenance of equilibrium is essential. In semi-erect bipedal animals, the tail helps to counterpoise the body. Kangaroos have and the Dinosaurs had a very powerful tail which acts as the ‘third leg’.
Reduction of the length of neck. In bipedal animals, particularly in mammals, there is always a tendency towards the shortening of the neck to bring the head near to the shoulder. The number of the cervical vertebrae is usually seven but the cervical vertebrae coalesce into a rigid body.
(b) Modifications for Food-Getting:
The lengthening of the limbs in cursorial animals takes the head sufficiently away from the ground. So to get food and drink, the neck and the skull become elongated. The elongation of the neck and the skull bear a definite ratio with the lengthening of the limbs, which is clear from the phylogenetic stages of the horses already discussed in Part One of this volume.
Type # 2. Fossorial Adaptation:
Toget more food and safe shelter, animals tried to go beneath the surface of the earth to lead the subterranean life. Fossorial adaptation shows different gradations and as a consequents different degrees of structural modifications are encountered to tunnel the soil. Animals living above ground may dig the soil for search of food.
In these forms the structural modifications are limited to the specialisation of the digging device only. The tusks of the elephants and the snout of the swine serve as digging’ machine. Although structural alterations are not so great, the associated structures become modified.
In elephants the development of the incisors into tusks causes tremendous modifications of the skull, and jaws. But the animals which dig the soil for retreat exhibit greater modifications.
In this category we find two gradations:
(a) Fossorial animals dig for retreat but had to come over the surface for food.
(b) Fossorial animals dig for retreat and find their food also beneath the surface. The animals which come under this group are wholly fossorial and show greatest specialisations.
(a) Modifications for Fossorial Life:
Fossorial animals have moulded their body to offer least resistance to dig the soil. The body becomes either spindle-shaped or fusiform which helps to penetrate the medium very easily. The greatest diameter of the body lies near the shoulder. The shoulder girdle Is strongly built.
In Echidna the greatest diameter of the body lies further backwards. Extreme modifications are observed in snakes, Gymnophiona and in some lizards where the body becomes very much elongated, cylindrical and limbless. The tail becomes invariably short in fossorial animals. In extreme cases the tail is vestigial which may serve as a tactile organ. The sense organs have the general tendency towards reduction.
The eyes and the external ears tend to become vestigial. The reduction of these structures correlates with the degree of fossorial adaptation. Complete subterranean life causes total visual reduction. The eyes are very small and insignificant in the members of Bathyergidae and Geomyidae. In Spalax the visual apparatus is a complete structure being represented by small specks.
The eyes are imperfectly developed and nonfunctional in marsupial mole (Notoryctes), vestigial in common mole (Talpa) and remain covered by integument in Chrysochloris. The pinna in burrowing animals may cause obstruction, so they tend to disappear.
The external ears are very small in Geomyidae, extremely reduced as fringe of skin in Bathyergidae. In most of the fully fossorial animals the external ear is entirely absent.
(b) Digging Apparatus:
Fossorial animals must possess efficient digging apparatus.
Snout—as the digging machine. The snout comprises an important device for digging in Heterodon (hog- nosed snake) and swine. The skull becomes subconical and lacks the expanded zygomatic arch. An extra bone (prenasal) may be developed at the tip of the nasal cartilage to aid in digging as observed in Talpa and swine.
Teeth—as the digging machine. The teeth, particularly the incisors and the canines in many animals, help in digging. The canines act as digging apparatus in swine. The incisors are usually forwardly protruded to facilitate digging. The tusks of the elephants are the effective digging instruments.
Forelimbs— as the digging machine. Extreme modifications and specialisations of the forelimbs are observed in the fossorial animals. These limbs constitute the most effective digging machine. The limbs show the tendency towards the reduction in length and have become strongly built, because shorter limbs have positive advantage during burrowing.
The forelimbs become not only short and strongly built but also they become very much broadened. They also differ materially from the hind-limbs. They have undergone divergent specialisation for the purpose of digging (Fig. 4.2).
The hind- limbs remain less modified and help to drive the animals forward. The digits of the forelimb are provided with strong claws. Broadening of the hands is effected by the addition of an extra bone in addition to normal number of five. This extra piece of bone is called the radius sesamoid or os falciforme. This increases the breadth of the palm in some moles and Echidna.
In Chrysochloris, the hand has four digits but its effectiveness is increased by the elongation of two middle fingers which are furnished with powerful claws. The long bones of the forelimbs are very strong and are provided with prominent ridges for the insertion of the muscles.
The proximal ridges of the humerus are highly developed to give attachment to the powerful shoulder muscle. The olecranon process is usually large to give attachment of the powerful triceps muscles.
Strengthening of the Shoulder:
Powerful shoulder is an asset for fossorial forms, because it has to withstand great pressure during digging. Narrow shoulder is another modification of the fossorial forms. In monotremes and northern moles, the entire shoulder girdle is shifted forward to the neck (Fig. 4.3).
In Chrysochloris the sternum is convex inward. In true mole the shifting of the shoulder is also caused by the elongation of the first sternal segment, which bears very short clavicle. In case of monotremes, the shoulder girdle is supported by a very large T-shaped episternum.
Hindlimbs as Driver:
The hindlimbs remain less modified. The femur is not stout as the humerus and the ridges are not so prominent. Only alteration is observed in the partial fusion of the tibia and fibula, and the greater development of the calcanetim.
In Talpa, a large bone is attached to the side of the tibia which can be compared with the os falciforme of the forelimb. The only function of the hind-limbs is to push the animal forward during digging. The ilium and ischium are situated parallel to the vertebral column and are greatly elongated.
Modifications of the Vertebral Column:
The vertebral column is rigid. The sacral vertebrae show the tendency of fusion. In Talpa and the hedgehogs, the vertebral column is strengthened by the development of intercentral ossicles. Several cervical vertebrae become either more elongated or show fusion.
Type # 3. Scansorial Adaptation:
Arboreal life is sought for safety, retreat and for abundance and easy procurement of food. From the phylogenetic history of the vertebrates, arboreal life is very significant, because the flying vertebrates, excepting flying fishes have been derived from scansorial vertebrate forms. Like fossorial forms, the scansorial types also furnish gradations of specialisations.
The first category includes the animals who are usually rock and wall climbers. Geckoes and house lizards are the typical representatives where the adaptation is limited to the development of climbing organs at the bases of digits. The flying squirrels, included in this group show also scansorial adaptation in addition to their volant modifications.
The second category comprises of many insectivores, rodents and carnivores which can climb the trees very easily as well as they are quite at home on the ground. They actually lead double life, in trees and on land.
The third category includes the animals who are fully arboreal and make the trees their home and may in rare occasions descend to the ground. These forms have undergone greatest modifications for living on trees.
The animals belonging to this category exhibit different modes of progression:
(a) Progression on the upper surface of the trees by the help of the fore- and hind-limbs. Most of the arboreal forms move by this method.
(b) Progression on the under surface of the trees by both the pairs of the limbs. The sloths are the typical examples. They possess large, recurved and powerful claws by which,they can remain suspended from the branches of the tree. They are also able to move upon the branches of the tree. Other forms, such as the bats and Galeopithecus may also be included under this category.
(c) Progression by swinging by the forelimbs is observed only in Primates. They can move with speed and accuracy by the forelimbs from tree to tree.
Modifications for Scansorial Life:
The general contour of the body and the limbs are extremely modified to lead perfect scansorial life.
For climbing, the chest along with ribs, the shoulder and the pelvic region are very strongly built. In fully arboreal types, the thorax is subcircular and the ribs are extremely curved. In Sloths, the ribs are many to afford the weight of the viscera in an inverted state.
Another modification is the elongation and the increment of the number of the dorsolumbar vertebrae. In a tree-sloth, Choloepus and in an arboreal rodent, Capromys the number is increased from nineteen to about twenty-five. In Dendrohyrax, six additional vertebrae are present.
In arboreal forms, the proximal portion of the limb becomes extremely elongated and bears a definite ratio with the climbing ability. The greatest modification is seen in Hylobates where the arms are very long and can even touch the ground while standing erect. The feet may also be modified as prehensile or grasping organs. The digits in this form may be opposable.
In those cases where the feet are not prehensile, the digits are furnished with powerful claws as seen in Squirrels. The claws may be curved. In Erethizon, a tree porcupine, in addition to recurved claws, the sole possesses spines which help in climbing. Presence of adhesive discs at the tip of the digits or on soles is climbing organs.
These are observed in tree-frogs, Geckoes and Dendrohyrax. In tree-frogs the sticky secretion of the adhesive discs helps in adhesion. In Geckoes, adhesion is caused by creating numerous vacua on the adhesive digits. In Dendrohyrax, adhesion is done by creating partial vacuum on the soles. The feet are modified as prehensile organs in marsupials and primates.
In marsupials the hallux and the fourth digit are opposable, while the second and the third being bounded by a common integumentary sheath, thus showing syndactyly. In Primates, the first digit is opposable. Another remarkable climbing modification is observed in Koala where each foot has long clawless great toes and clawed syndactylous second and third toes.
The fourth and fifth toes are also provided with powerful claws. In Chamaeleon, syndactyly is observed in both the limbs which are powerful grasping organs. In scansorial birds the hallux opposes the second and third toes and thus offers a very firm grip during perching. In Woodpeckers, Parrots and Cuckoos, the toes are provided with strong claws.
Although the digits are very vital in arboreal life, occasional reduction in the number of digits is observed in many forms. Greatest modifications are observed in Choloepus where the hand is provided with two digits and the foot has three digits. In Bradypus, three digits are present in each limb.
The pectoral girdle is strongly built. The clavicles and scapulae are highly developed. The clavicles play a very important role and can withstand the pressure of the breast muscles. The pelvic girdle is less modified. The ilia become broadened out to support the viscera.
Modification of the Tail:
In most arboreal forms, the tail is prehensile. In Chamaeleon the tail is prehensile. In non-prehensile forms, the ectodermal scales (as in Anomalurus, a flying squirrel) prevent the animal from slipping. One of the most remarkable examples is Ateles, the spider monkey, where the tail is highly prehensile.
Type # 4. Desert Adaptation:
Desert is a specialised terrestrial environment which has its quota of animals and plants living their life quite suitably by making numerous morphological and physiological adjustments. It is extremely difficult to define a desert, because various types of desert are encountered.
To name a few, these are:
(a) low-rainfall desert (sandy deserts)
(b) cold deserts (such as Polar and Alpine deserts);
(c) low- nutrient deserts caused by the excessive use of the soil;
(d) deserts due to toxication such as the adjoining areas near the volcanoes and
(e) high-salt deserts.
Of these types of deserts, the low-rainfall desert is usually regarded as the most typical one. The common feature of all these deserts is the presence of dryness where rainfall is usually less than 10 inches in a year.
Characteristics of the Low-Rainfall Desert:
It is generally regarded as a vast stretch of drifting sand with the following physical conditions:
(i) Low rainfall,
(ii) Extreme temperature,
(iii) Low water vapour in atmosphere resulted from excessive solar radiation,
(iv) Increased air movement which results into sand and dust storms and
(v) Poor vegetation excepting in rare cases, e.g., oasis. In this portion of the land evaporation of water exceeds its condensation.
Deserts have their own inhabitants which show peculiar modifications for desert life. To the desert forms two primary necessities stand on the way. They are moisture-getting and moisture-conservation. Defence against physical and organic environments is also a very important factor in desert life.
Modifications for Moisture-Getting:
One of the primary requisites for desert life is to get adequate moisture. The desert forms have developed a number of devices to obtain moisture which is really a rarity in desert. In desert, moisture suspended in atmospheric air is not showered as rain but it may form only dews which are utilized by diverse means. For animals living in desert the main source of water is the juices of plant or may be the blood of the prey.
Desert animals have developed the habit of occasional drinking of water and in some cases the collected dews taken along with plant food is sufficient to quench the thirst. The skin of Moloch horridus is hygroscopic which has the property of absorbing moisture from the atmosphere like that of blotting paper.
Modifications for Moisture-Conservation:
Conservation by Storage:
The primary method of conservation of moisture is done by utilizing little quantity of water in metabolism. It was previously believed that camel, the ship of desert, possesses numerous pouch like diverticula, called the water cells in the stomach which store water for the emergency use in desert.
But this belief is not acceptable now (vide J. E. Hill, 1946, Natural History, 55: 387). These pouches are not sufficient to hold as much water as the animal would require during a prolonged journey through sandy desert where no water is available. The water cells may sometimes be filled with water which are generally metabolic water collected from the different parts of body to moisten the food to facilitate digestion.
Some quantity of water is stored in the connective tissues and muscles. This quantity of water usually comes from the breakdown of glycogen stored in muscles and of fat present in the hump. It has been estimated by physiologists that every 100 g of fat oxidised in the body yielded 110 g of water. An estimated quantity of 8 gallons of water may be produced from the breaking down of fat stored in the hump alone.
Conservation by avoidance of evaporation:
The desert animals can conserve moisture by being non-perspiring. Perspiration or sweating is a physiological process in mammals to keep the body cool, but in desert non-perspiration is a special adaptation.
Modifications for Defense:
Protection from hostile environment is necessary for desert animals. One of the notable unfavourable physical conditions prevalent in desert is the extreme temperature.
Protection from Extreme Temperature:
In sandy desert the day temperature is extremely high and the mercury column may touch 182° Fahrenheit, but the night is cold and the hot wind does not blow. The desert animals usually seek the shadow of great rock in daytime or they can burrow the sand to escape the extreme cold or heat.
The desert insects can withstand considerably high temperature. They can sit or move comfortably on hot stones which are unbearable for other animals.
Protection from Sand:
Protection against the harmful effect of sand on the vulnerable parts of the body is of primary importance. So the eyes, nostrils, cars are extremely modified to fight the sand storm.
For security and for searching suitable place for shelter, acute vision is necessary. In camels, the eyes are large and are guarded by plenty of long eyelashes. In a burrowing desert snake, Typhlops, the eyes are small and are over-hanged by head-shields.
In some lizards, Agama and Phrynocephalus the edges of the eyelids are broadened into plates beset with scales. The upper eyelid is enlarged in some cases as observed in Teratoscincus.
In some reptiles the eyelids are extremely modified. In Mabuia the lower eyelid is greatly enlarged with a transparent window, through which the animals can see when the eyes are closed. Extreme specialisation is found in Ablepharus where the whole of the lower eyelid is transformed into a transparent membrane which is united with the upper eyelid.
The nostrils are directed upwards in most desert reptiles. In many desert snakes the nostrils are protected by complicated valves and may be reduced to small pinholes. In camels the nostrils can be closed. Saiga tartarica possesses a large inflamed nose and the narial apertures are placed far backward.
In desert reptiles the ear opening is very small and may be protected by scales. In extreme cases the opening may be absent. In camels the openings of the ears are protected by hairs. The greatest modifications are observed in camel and ostrich where the head is placed on a very long neck. This brings the vital sense organs as for as possible above the sand level.
Protection against Enemies:
Desert animals have adopted various devices to protect against the predators. These are: Colouration. The hues of the desert animals are such that they can harmonize their body colour with the colour of the sand.
One of the typical examples is Gazelle which matches the surrounding sands and stones to such an extent that it becomes impossible for the predators to detect the animals when at rest. The prominent yellow and black colouration of Gila monster, the only venomous lizard, shows warning colouration.
Modifications of Body Surface:
The dry, hard and spinous surface is the characteristic of many desert animals. The spinous scales of the body of Moloch horridus and the large spines on the body of the horned toad, Phrynosoma (Fig. 4.4) are the two typical examples.
Possession of the venom is an attribute of desert adaptation. Many desert animals are venomous. The desert snakes, Gila monster, Ants and Spiders are some of the poisonous desert animals. The common red desert ants, the rattlesnakes and all the spiders in desert are dreadly venomous.
Tarantula is a venomous and dreadly spider. The Scorpions are also highly venomous. Odour. Repulsive odour is also an adaptive feature in many desert forms.
Modifications for Speed:
The desert animals usually move with high speed, because they have to travel far off in search of food, drink and safety. The sand-adapted feet of camel serve as a very effective locomotor organ.
Type # 5. Volant Adaptation:
Volant animals constitute a very interesting and highly specialised group of vertebrates. They have undergone extensive adaptive radiations and have beautifully modified their body to lead a perfect aerial life.
Duality in Volant Adaptation:
Volant adapted forms are not such as the term denotes, but they must return to the surface of the earth for rest.
All the volant forms exhibit two-fold adaptations:
(a) Volant adaptation for flying in air and
(b) Cursorial or Arboreal or aquatic adaptation for the surface life.
As a consequence two directional structural adaptations have occurred. The most illustrative volant forms are the birds, where the forelimbs have modified into wings for volant life and the hindlimbs are specialised for cursorial and/or arboreal or aquatic life. In true volant forms, the modifications for flight in most cases have superseded other adaptations.
Modifications for Volant Life:
In discussing the modifications for volant life, the discussion is restricted only to the structural adaptations for flight as the other adaptations such as the cursorial or arboreal, have been discussed previously. In volant forms the cursorial or arboreal adaptation is quite typical. In aquatic forms the digits of the hind-limbs have become webbed to aid in swimming.
Prerequisites for Volant Life:
For living in air an organism must possess the following unavoidable requisites:
(a) Organs for flight;
(b) Lightness and rigidity;
(c) Energy and power;
(e) Balancing and
These are the trump cards upon which the avian organisation is based.
The degree of structural modifications in aerial forms depends upon the ability of flying. In all the flying forms the contour of the body tends to modify in such a way as to offer least possible resistance in air. Amongst the vertebrates two types of flight are encountered.
(i) Passive Flight:
This type of flight is also called the gliding flight, when the animal takes a leap from a high point which is either the tree or the peak of mountain. After taking the initial leap, the animals can glide to a lower region and thus cover some horizontal distance. During such gliding the main role is played by the gravity’ and in most cases is helped by sustaining structures.
(ii) True Flight:
This type of flight is caused by the activity of the organs for flight and the organisms have the power of sustained movement through the air.
Organs for Flight:
Expanded Pectoral Fin:
The inclusion of the flying fishes as the true flier is questional. There are several genera of fishes which can fly. The most illustrative example is the Exocoetus where the pectoral fins become greatly expanded as the organs for flight.
The pelvic fins are comparatively small. The lower lobe of the tail fin is much longer and can accelerate the speed. The length of the flight can exceed 200 to 300 yards. The other flying fishes are the various species of Dactylopterus with beautifully coloured wings, Pantodon of Africa, Gastropelecus of British Guiana, Pegasus volitans of Japan, India, China and Australia.
Webbed Feet as the Sustaining Surfaces:
The only volant adapted amphibia is the genus Rhacophorus whose digits of the limbs are webbed. The expanded web helps the animal to have a prolonged leap although the rudiments of patagia are present in front and behind the arms. Amongst the tree-frogs, Rachopkoms pardalis has rather greater power of gliding.
Patagia as the Sustaining Surfaces:
Patagia are primarily folds of skin expanding from the body and may be supported by various skeletal elements. In a lizard, Draco (or flying dragon) the sides of body become extended outward to form two large membranous surfaces supported by five to six elongated ribs. The patagia can be folded against the body when not in use.
In another form, Ptychozoon, the patagia are present along the sides of the neck, body, tail and limbs. In birds, traces of patagia may be present (prepatagia and postpatagia) in front and behind the arm. The development of feathers in birds has led to the degeneration of the patagia.
In mammals, particularly in Galeopithecus (or ‘flying lemur’) the patagium extends not only between the fore- and hind-limbs but is abo present between the neck and the forelimbs, between the tail and the hind-limbs and even between the digits as webs.
Wings as Active Organs for Flight:
Wings are the greatest anatomical deviation recorded in the vertebrates for flying. Wings are complex organs consisting of several structural elements. Three -types of wings are observed in vertebrates (Fig. 4.5).
(a) Wings in Bat’s:
The humerus is well formed, the ulna is vestigial but the radius is very long and curved. The pollex is free and clawed. The other digits are extremely elongated and support the patagium. In Megachiroptera the second digit is independent and bears a claw. But in Micro- chiroptera the second and the third digits are attached distally and are devoid of claws.
(b) Wings of Pterodactyl:
The radius and ulna are almost equally developed. The fourth metacarpal is heavily built and bears the large wing-finger. The other three metacarpals are slender and support the other three digits which are independent and clawed. The wing-finger supports the entire anterior region of the patagium.
(c) Avian Wings:
The forelimbs are modified into true wings which are composed of bones, muscles, nerves, blood vessels and feathers. In modern birds, the wings have the framework of bones. The humerus is large, strongly built and possesses prominent ridges for the attachment of flight muscles. The radius and ulna are slender, stout and slightly curved. Only three phalanges are present and there is no trace of the fourth and fifth digits.
The bones of the wrist, hand and fingers are extremely specialised both by the loss of bones or by the fusion of bony units. In modern birds, the claws are lacking in the digits, but in fossil birds and in some ratites claws may be present. The patagia are rudimentary as their functions are taken over by the feathers. The feathers are borne by the wings. The feathers are called the remiges.
Overlying the basal part of the remiges there are several rows of coverts which close the interstices between the remiges to make the wing a continuous surface. The muscles are highly developed. Several flight muscles such as the pectoralis major, pectoralis minor, coraco-brachialis, tensores patagii and many others are intimately associated with the wing and regulate the activities of the wings.
Modifications for Lightness and Rigidity:
Many elements combine to make the volant animals the models of mechanical perfection. Bones are stout to withstand the pressure of flight muscles during flight. Bones of the pterodactyls and birds are hollow and pneumatic.
The climax is reached in birds, especially in carinates, where the development of air-sacs; fusion of the vertebrae; loss of internal organs such as the gall bladder, urinary bladder, loss of right ovary and the vestigial right oviduct in many birds makes the body much lighter.
In birds the skull bones are paper-like thin and are sutureless. Although in birds the bones are pneumatic and thin, the bones have a secondary plastering to make them rigid.
Modifications for Obtaining Extra- Energy and Power:
The capacity to convert chemical energy into mechanical motion through the combustion of fuel is the secret of attaining sustained power. Greatest modifications are observed in birds.
The lungs are proportionately smaller in size but the efficiency is increased by the development of the air-sacs which also help to send oxygen directly to many tissues. In birds die body temperature is rather high which hastens the combustion.
Non-conducting coat of feathers over the body prevents surface loss of heat. Crude power as food is stored inside the body in the crop. The heart is proportionately larger in size and the circulatory system as a whole is very efficient. The physiological efficiency of respiration is increased by the development of air-sacs in birds and chameleon.
Modifications for Obtaining Speed:
Speed is a must for volant life. The body is trim-built or fusiform and without any extra resisting projection. In carinates the existence of air-sacs between the flight muscles help in flight by reducing friction.
Modifications for Balancing and Steering:
To equalise the air-pressure on the wings, the surface of the wing can be decreased or increased. In carinates the air-sacs are so arranged that the proper centre of gravity can be maintained by shifting the air from one side of the body to the other. The cerebellum in bird is very well-formed. The tail is also provided with the rectrices which as a whole helps steering during flight.
Modifications for Control:
Especially in case of birds, the brain is well developed with well-formed corpus striata. The optic lobes are well-formed and show the hint of quadrigemina. The eyes are provided with pecten which increases the acuity of visual perception. The eyes in case of pterodactyls were also very well developed and large.
The eyes were provided with sclerotic plates. Bats have good vision, especially in twilight. They have well- developed tactile sense. The ear and the facial appendages are the main sensory seats to prevent the crush with the unwanted approaching objects. The patagia in bats are also very sensitive.
In birds, the neck is usually long. Birds have a wide range of vision all around, they can rotate the head to 180° due to the possession of heterocoelous cervical vertebrae. The jugular veins are also very long and form a loop at the anterior end.
Type # 6. Aquatic Adaptation:
Almost all the classes of the vertebrates have representatives who lead aquatic life. Two types of aquatic vertebrates are encountered, the primary aquatic vertebrates and the secondary aquatic vertebrates. The primary aquatic vertebrates are the fishes that have evolved from aquatic progenitors.
The fishes are perfectly adapted to aquatic environment. Besides the fishes, many lung-breathers have gone back to the primal aquatic home for food and safety and exhibit extreme modifications for aquatic life.
Requisites of Primary Aquatic Adaptation:
The adaptation of fishes to a watery medium is perfect. So the structural adaptations of the fishes may be taken as the requisites for aquatic adaptation.
The body is compressed into a stream-lined form with no extra protuberances that may cause obstruction for swift movement through water.
Locomotion is primarily effected by the lateral undulations of the body. The fins also help in the process and are regarded as the accessory locomotor organs. Of the fins, the tail fin is the most important propelling organ. In addition to the unpaired fins, the paired fins, comparable to tetrapod limbs, are present and serve, in addition to normal function, as stabiliser.
Some sorts of hydrostatic mechanism are necessary for aquatic life. Almost all the fishes excepting the elasmobranchs have a gas-filled swim- bladder.
Structural Modifications in Secondary Aquatic Vertebrates:
The lung-breathers, due to the stress of circumstances, have been compelled to go back to the water. As a result of living in watery home they exhibit extensive modifications. The greatest difficulty which the lung-breathers had to confront was the inability to breathe in water.
Modifications for aquatic life involve many anatomical and physiological alterations. The entire modifications can be marshalled into two perspectives, securing of food and the attainment of swift passage through water.
Internal and external alterations are so profound in quality and quantity that one may be at a loss to interprete them properly. The amphibians constitute a transitional group to furnish partial aquatic adaptation.
The structural modifications in them are limited to the possession of webbed feet, laterally compressed swimming tail which may have fin-like outgrowth along the upper border. They can respire by gills in adult stages, but the lung-breathers are numerous. But the greatest modifications are observed in aquatic reptiles, birds and mammals. They furnish typical examples of adaptive convergence.
Modifications for Locomotion:
Adaptations connected with the body forms:
There is always a tendency to eliminate any unevenness of the body contour that would offer resistance during progression through water. Reduction or absence of neck in whales, dolphins and manatee brings the head near to the thorax. In cetacea, sirenia and pinnipedia the greatest breadth lies in the anterior one-third of the body. The body becomes stream-lined.
The tail becomes enlarged and the mobility’ of neck is lost in all the fully secondary aquatic forms. In Protomogale, Limno- gale and River-otters the body is fusiform, but in Zeuglodonts the body is not fusiform but is largely anguilliform.
The chest tends to become cylindrical and is modified to bring the visceral cavity higher towards the back and thus insuring greater stability in floatation and also increases the lung capacity. The ribs are highly arched dorsally and move upward.
Modifications in ‘Oar Propulsion’:
In case of oar propulsion a. gradation of modifications of both the limbs is encountered. In Turtle, Plesiosaurs, Walrus, the limbs are equivalent in size.
Forelimbs and Pectoral Girdle:
In highly- specialised aquatic mammals the humerus has become shortened. The clavicle is also absent in Pinnipedia, Sirenia and Cetacea. The scapula is elongated for the attachment of muscles. The digits are webbed in Pinnipedia and Ornithorhynchus and the degree of webbing varies greatly. In Walrus, the digits are folded and are covered by skin, but the individualities are retained.
In Sirenia, the development of paddle is seen, where the mobility of the various joints is lost and the entire skeleton is enclosed by skin showing no external division of phalanges. In Cetacea, the forelimbs are modified into flippers and show both hyperphalangy as well as hyperdactyly. In Dolphin, the length of two or three digits is increased while the others are reduced.
Hind-limbs and Pelvic Girdle:
The hind-limbs in water opossum (Chironectes), Protomogale, Hippopotomus, Phocidae, Odobaenidae are the chief and/or exclusive swimming organs. In Ornithorhynchus and Otariidae the hind-limbs act as equilibrators.
The general tendency of the hind-limbs and the pelvic girdle is to disappear. In rare cases, small bony vestiges representing the pelvis and limbs may be seen buried deeply in the muscle.
Modifications for ‘Caudal Propulsion’:
The tail is used as a rudder and for the purpose of keeping the head downward when the animals search food in the mud. The tail is modified for two fundamentally different principles, especially in aquatic mammals. The tail becomes narrow horizontally and broadened in vertical plane.
This condition is observed in Hippopotomus, Protomogale, Nectogale where the tail is narrow. The tail is rounded in Chironectes. The tail is flattened vertically and broadened in horizontal plane. It is single-lobed and rounded in Manatee. The tail may be notched in whales, Dugong and Sea cow (Rhytina). In Cetacea and Sirenia, the tail has two flukes.
The caudal fin of marine reptiles is quite different from that of aquatic mammals. It is horizontal and the terminal part of the vertebral column equally supports the flukes. In secondary aquatic vertebrates, the unpaired fins, if developed, are not supported by skeleton.
The dorsal fin is triangular in Ichthyosaur. In many whales, the dorsal fin is entirely lacking, but in Balaenoptera musculus it is small and situated well off upon the tail.
Modifications for Physiological Regulation:
Modifications of the Integument:
In aquatic vertebrates the integument is extremely modified. Loss of armour has occurred in Ichthyosaur. In majority of aquatic mammals, loss of hair is seen. In Cetacea and Sirenia, there is no trace of hair\excepting few bristles around the muzzle.
The hairs may be absent even in foetus in White whales and Narwhales. Acquisition of a subcutaneous layer of fat (blubber) in aquatic mammals compensates the loss of hairs by helping in the retention of body heat. The sweat and sebaceous glands are also absent.
Modifications for the Protection of Sense Organs:
Olfactory Sense Organ:
In case of aquatic vertebrates, the external nostrils have shifted towards the apex of the head. The nostrils are closed involuntarily and opened voluntarily as in Cetacea and Sirenea. The closure is done by the action of the maxillonasolabialis muscles and the apertures are further complicated by the development of pads and valves. The aperture is crescentic in Sirenea.
The valves guarding the narial apertures close at the end of inspiration and open at the initiation of expiration in Cetacea. In Odontoceti the external narial aperture is single and medially placed. The whole olfactory apparatus is appreciably reduced in Pinnipedia.
Acoustic Sense Organ:
In case of aquatic mammals, there is a general tendency for the elimination of the external ear. The opening of the ear may be closed which is usually accomplished by pulling into the orifice of a valvular plug. In Phocidae the external auditory aperture is small and pinhole-like. In Cetacea the ear bones are extremely hardened.
Visual Sense Organs:
The eyelids show the tendency of degeneration in aquatic vertebrates, especially in mammals. In Cetacea the eyelids are absent and the nictitating membrane is present over the eye-ball. The eyes become adapted to aquatic vision. The visual organs exhibit great optical adaptation. The lens is spherical and the refractive index is high.
Unusual number of rods are connected with a single cell and tapetum lucidum is extensively developed. The other adaptations are the reduction in the size of cornea, the unusual development of choroid and perichoroidal lymph spaces. The cornea is laterally thickened and the sclera is also greatly thickened. The glands are comparatively well developed to produce oily secretion.
Modifications of Soft Parts:
In case of aquatic forms there is a general tendency for the teeth to become simplified and become sharply pointed for grasping the slippery prey. The teeth may be absent in one jaw as in Sperm-whale, but in Baleen Whale both the jaws are devoid of teeth. The function of the teeth is taken up by the baleen plate. The molars of Platypus are shed at a very tender age and their places are taken up by the horny plate.
Although well-toothed as a rule, Ichthyosaur showed reduction of teeth in some cases. The buccal cavity in Cetacea is Very large and the stomach becomes complicated by the formation of chambers. The salivary glands are absent or reduced in aquatic mammals.
In Porpoise, the lungs are provided with cartilaginous armature to give unusual strength and incompressibility. In Cetacea and Pinnipedia, the intrinsic musculatures of the lungs and the disposition of the diaphragm enable to emtify the lungs more completely. The lungs are spacious. In Cetacea and Sirenea, the diaphragm has become horizontal in position.
In most of the aquatic mammals, especially in Cetacea and Sirenea, the blood is comparatively rich in haemoglobin. Another peculiar feature is the presence of retia mirabilia in Cetacea, Sirenia and Phocidae.
Particularly in case of aquatic mammals, there is a tendency to eliminate the scrotum. The testes are not actually abdominal but are situated in a pouch near the inguinal ring. The offsprings are precocious.
Disposition of Mammae:
In Cetacea and Sirenia there is one pair of mammae situated inguinally. In some Sirenea, the paired mammae are axillary and situated practically upon the posterior border of the flippers. In Myocastor, there are two pairs of mammae situated almost upon the back, one pair is located upon the shoulder and the other pair is situated near the haunches.
Modifications of Hard Parts:
The floatation and the degree of water pressure have exerted tremendous influence in modelling the shape of the Cetacean skull. In aquatic mammals, there is a tendency of shortening the cranium and the facial portion of the skull tends to become elongated. The zygomatic arch is greatly reduced to vestige as seen in Cetacea.
In Dolphins and whales, the posterior portion of the skull is globular and the anterior portion is prolonged into a rostrum-like structure. In Sirenia the bones are compact and are heavily built for getting submarine vegetation. But in Cetacea and other swimming forms the bones are made light with spaces filled with fats. In Mystacoceti the bones of the skull are telescoped at the occipital region.
The vertebral column tends to become simplified. The cervical vertebrae are ill-developed and become condensed. In whales, the number of cervical vertebrae is seven but becomes fused into a compact bony piece.
The articulating processes of the vertebrae in the trunk region become reduced but become elongated in the tail region for the attachment of the tail muscles. The sacrum is undifferentiated in aquatic mammals and the sacral vertebrae lose their identities.
The morphological as well as the physiological alterations are most striking in aquatic animals. Moulding force is so severe in case of aquatic adapted forms that the identity may be lost and one finds perfect convergence of bodily forms between diverse groups. All the secondary aquatic forms have evolved from terrestrial ancestors.
Although this fact is not supported by palaeontology, still the subsequent stages can be reconstructed with considerable accuracy. The terrestrial forms may have been forced to come to water for food and for enemies that pursue them.
On coming to water they may again be confronted with aquatic enemies and as a result they are forced back to land. Some of them may encounter favourable condition in water and relinquish the terrestrial home and eventually become highly specialised aquatic animals.
Type # 7. Adaptations in Cave-Dwellers:
Caves are essentially the abandoned channels of underground rivers in the limestone regions. The caves also provide homes for animals. Denudation of the surface by external agencies may produce caverns where ingress of cave-dwellers occurred.
On coming to caves, the animals became isolated from others with suitable adaptations for living in caves. Like the deep-sea forms, the cave fauna are relatively recent in origin and the adaptive changes undergone by them are mainly degenerative specialisations.
Physical Conditions in Caves:
Absence of sun-light and uniformity in temperature are the two striking physical characteristics existing in caves. The caves may be divided into the following zones depending on variable physical conditions. Twilight zone. This zone comprises of the mouth of the cave and a little amount of light may penetrate.
This zone constitutes the transitional region of the cave. Fluctuating temperature zone. Seasonal or diurnal variation of temperature is felt in this zone. Inner cave region. In this zone light is absent and the temperature remains more or less constant.
The structural modifications of the cave- dwellers are mainly due to lack of light, scarcity of food and changelessness physical conditions. Two categories of cave- dwellers are seen, the temporary visitors to the caves and the permanent inhabitants. Amongst the temporary visitors to the caves the instance of brown bat (Myotis lucifugus) is worth mentioning.
The adaptations in permanent cave-animals are quite striking. The principal modifications are: the bleaching of pigmentation of the skin; the reduction of eyes and development of highly sensitive tactile organs, the scarcity of food has modified the organs of digestion, as a result the body becomes slender with delicate appendages.
Characteristics of Permanent Cave-Dwellers:
Many fishes inhabit the caves and show structural modifications. The catfish (Gronias nigrilabris) is almost blind. Majority of the cave fishes show sign of degeneration of eyes. In a small cave fish called the Typhlichthys sub- terraneus, the eyes are present in young stage but become useless in adult. Amblyopsis spelaeus is totally blind but possesses well-developed tactile sense organs.
Amongst Amphibia, a large number of salamanders live in caves permanently and have undergone degeneration (Fig. 4.6). Typhlotriton spelaeus has normal eyes in larval condition but become degenerated in adult. The eyelids are fused and the retinal layer lacks rods and cones. In Typhlomolge rathbuni, the eyes are devoid of muscles.
The eyes are entirely covered by skin .and non-functional. The limbs are extremely elongated and slender. Proteus anguinus is another peculiar cave amphibian where the body is white and lives in complete darkness. It can exist for a long time without food. It is completely blind but the body is experimentally seen to be very sensitive.
Amongst the amniotes there is no true cave-dweller. Although the structural modifications in some burrowing lizards and snakes are same as the cave-dwellers, these cannot be due to cave-dwelling. Amongst the birds there is no permanent cave-dwelling form.
Only one mammal, Peromyscus leucopus (white-footed mouse) exhibits a sort of transitional adaptation between epigean and cave-dwelling forms. They possess protruded eyes and long tactile whiskers.
The environment in caves is non-fluctuating and is more or less uniform. This changelessness has made all the cave-dwellers to look alike. The cave- dwellers are weak and degenerated forms. To escape from the epigean enemies, they were forced to take the resort in caves for retreat. Absence of light and lack of food have caused the cave-dwellers towards degeneracy.
Type # 8. Deep-Sea Adaptation:
Of the bathymetric realms, the most important and significant one is the Holo biotic or Marine realm. This realm is again divided into four characteristic sub- realms, such a Strand, Shallow sea, Pelagic and Abyssal zones. The abyssal realm includes all the waters below the depth of 10G fathoms.
The abyssal realm is divided again into the abysso-pelagic and the abysso-benthonic zones. The abysso-pelagic zone has no substratum and the animals have pelagic characteristics, while the abysso-benthonic zone has a substratum and the animals inhabiting in this region are extremely modified and peculiar.
Physical Characteristics of Deep Sea:
Four remarkable characteristics prevail in this area.
They are as follows:
(a) Absence of Sunlight:
The limit of penetration of sun’s rays is about 200 fathom. Beyond that depth there is no sunlight.
Because of depth the movement of water is almost absent and the movement is exceedingly slow.
(c) Cold Temperature:
Beyond a certain depth the temperature is nearing to freezing point and the temperature remains constant. Diurnal and seasonal fluctuations of temperature cease.
The pressure of the water column is enormous and increases with the depth.
(e) Lack of Green Vegetation:
Besides the above four factors, another factor is the total absence of green vegetation due to absence of sunlight.
General Trends of Adaptation in Deep-Sea Animals:
For leading deep-sea life, the animals had to confront the adverse physical conditions stated above. As a result they exhibit certain structural variations.
The general trends are summarised as:
(i) The deep-sea animals are weak and delicate.
(ii) The hue of the body is generally simplified.
(iii) The deep-sea forms are either with powerful telescoping eyes to catch maximum possible volume of light rays or are totally blind.
(iv) Development of long feelers to act as tactile organs,
(v) Almost all the deep-sea forms are luminescent.
(vi) Most of the deep-sea fishes live on the exudes of decaying matters that have led to the loss of masticatory power. Others are seen to possess powerful jaws.
(vii) Most of them have wonderful devices for caring the youngs and the others produce enormous number of youngs to overcome the hostile environment,
(viii) Small is the another characteristic of deep- sea living.
Structural Modifications of Deep-Sea Invertebrates:
Almost all the phyla have representatives who lead deep-sea life. Amongst the invertebrates, sponges, corals, hydroids, few echinoderms such as brittle-stars and stalked crinoids, tube-dwelling annelid worms, some brachiopods, cephalopods, some arthropods like the barnacles are some of the deep-sea forms.
The modifications of the invertebrates are quite diverse and the description will be limited to the modifications of the vertebrates.
Vertebrates of the Deep Sea:
Amongst the elasmobranchs, the true sharks do not exhibit deep-sea characteristics excepting the luminous sharks (Spinax niger). The silver sharks or Chimaeroids show deep-sea appearance in having huge eyes and a long attenuated body and tail.
Of the teleost fishes, the typical deep-sea forms is the Cetomimus which has a large mouth, small teeth, very small eyes and scaleless body. But in another form, Ipnops there are no eyes and two large luminous organs are present on the head.
The family Stomiatidae is characterised by the possession of scaleless body and well-developed luminescent organs. The scales may sometimes be present but are extensively delicate.
In Gastrostomus, the body is long, slender and rows of luminous organs are present on the lateral sides of the body. The mouth is bounded by very large jaws. The cod-like forms (Gadiformes) have reduced mouth and dentition. The eyes are extremely large, the trunk is reduced and has a filamentous tapering tail.
The most remarkable group of fishes is the anglers which show typical deep-sea characteristics. The paired fins are adapted for crawling on the bottom of the sea and the anterior fin-rays of the dorsal fin function as a lure.
In Linophryne the fin-rays are provided with luminous organs to attract the prey. Another species, Oneirodes, is blind but has luminous organs to compensate the loss of eyes. In another deep- sea fish, Protostomias, specialised light producing organs are present as rows on the lateral side of the body.
Another striking feature is encountered in deep-sea fishes. Due to enormous pressure of water column, the body in flat fish becomes flattened and the mouth is shifted to the lateral side of the body.
The structural modifications in deep- sea forms are due to the peculiar physical conditions of the deep sea. The deep-sea forms are geologically very recent in origin. These forms were originally the inhabitants of the pelagic or littoral regions which migrated to the deep sea and became adapted there. Due to the changelessness of the physical conditions in deep sea the evolutionary possibilities are very low.
Type # 9. Parasites and Parasitic Adaptations:
Parasitism in wider sense is a specialised mode of living and the parasites in their efforts to survive have made innumerable adaptations and compromises. From the evolutionary point of view, parasitic existence is not actually a phenomenon of degeneration, but offers a chance of living in this crowded and in hospital world.
The parasites in course of time have established a unique harmonious co-existence with the host by the process of evolutionary adjustment. The ecological aspect of parasitism has been dealt in detail in Part Three of this book.
Factors Necessary for Survival of the Parasites:
The parasitic existence of the parasites depends upon finding a suitable environment in which they can live and propagate. Of other factors, the following four seem to be most vital.
(a) Provisions for successful admission to the body of the host.
(b) Suitable environmental conditions in the host.
(c) Protection against the normal metabolic processes of the host.
(d) Absence of reaction on the part of the host that may interfere with the normal metabolic activities of the parasites. The host should be tolerant and resistant.
Adaptations of the Parasites and the Host:
Parasitism leads to adaptations in the parasites as well as in the hosts that house them. Profound structural and functional modifications are encountered due to parasitism.
Adaptations in the Parasites:
The modifications in the parasites show two contrasting features, some structures become simplified while others become highly specialised. The modifications in parasites depend on the degree of parasitism.
The greater the degree of parasitism the more pronounced is the departure from the normal pattern. Gradations of adaptations are observed in parasites. Structures are slightly altered in the ectoparasites but they become highly modified in endoparasites.
Loss of Unused Structures:
Some unused structures are totally or partially eliminated. They are the organs of locomotion which become reduced or atrophied; reduction of the central nervous system and sense organs; gradual obliteration of the alimentary canal; reduction of the excretory system, etc.
Modifications of the Epidermis:
The epidermis is modified for protection and for absorbing nutrition as seen in parasitic helminths. Exoskeleton is totally lost. The organs of feeding often become more specialised and are better developed to suit the purpose. Food storage devices may be better developed in ectoparasites and the posterior portion of the alimentary canal may atrophy due to lack of residual matters.
In tapeworms, the alimentary canal entirely disappears. Most of the helminths lack alimentary canal, because they are constantly bathed in a medium of digested or semi-digested medium. Simplification and eventual loss of digestive system indicate the increasing dependence on the host for food and nutrition. Development of organs for attachment and penetration to the hosts are highly developed.
The modifications for the purpose of attachment are the hooks, spines, acetabula or sucking organs of the helminths. Modifications for the purpose of penetration are the glands in the larval forms of trematodes, in the vicinity of mouth of adult helminths are the devices for penetration into the body of the hosts.
Modification of the reproductive organs is aimed to produce prolific number of the offsprings to overcome the obstructions.
The eggs are usually produced in astronomical number. Occasionally self-fertilization occurs. In extreme cases completion of life-history needs intermediate host. The chance of survival is increased by the development of resistant envelope to seal the content of the egg from heat, drying and injurious influences. Such phenomenon also helps in the dispersal of the progenies.
Adaptations in the Host:
The hosts that provide shelter to the parasites, also exhibited certain adaptations. The reproductive glands are in most cases affected and in extreme cases may lead to degeneration. The reversal of sexes occurs in many hosts. In some extreme cases maturation and completion of the life-cycle is not possible.
Parasitism—a Case of Degeneration or Specialisation?
Parasitism furnishes one of the most significant examples of the power of adaptation in organisms to live parasitically. They exhibit all the possible far-reaching adaptations for the specialised environment prevailing within the body of the hosts. In some cases the parasitic adaptations are so severe that they diverge widely from the normal types of the families they belong.
As a method of adaptation for survival, parasitism must be regarded as a remarkably successful way of living, although the evolutionary trends may often culminate into retrogression and resulted into degeneracy.
The degree of degeneration may often depend on the degree of parasitism. Although the parasites in their efforts to overcome obstacles may become degenerated, parasitism has got great survival value in this congested world.
Evolutionary Value of Parasitism:
Nothing is definitely known about the origin and evolution of parasitism. But it can be said beyond any doubt that parasitism is more of recent origin than the free-living forms since free-living forms must have existed before the parasites could obtain hosts to live. Parasites, like other organisms, have been subjected to the evolutionary dynamics and their study can furnish valuable information about organic evolution.
Ectoparasites to Come First:
Ectoparasites were the first instances of parasitism and exhibit different degrees of gradations,
(i) The larvae are parasites but the adults are free-living as seen in the Glochidium larva of bivalved molluscs,
(ii) The adults are ectoparasites but the larvae are free-living such as the leeches and lampreys,
(iii) Both the adults as well as the larvae are ectoparasites as seen in Mallophaga.
Endoparasites Reach the Pinnacle of Parasitism:
The endoparasitic existence is the extreme case of parasitism,
In the endoparasitic forms almost same types of gradations are observed:
(i) The adults are free-living but the larvae are parasites as seen in certain forms of hymenoptera.
(ii) The adults are parasites but the larvae are free-living as seen in certain copepods and cirripeds.
(iii) Endoamoebae, malarial parasites, tapeworms are parasites all through their life.
Ectoparasites evolved before the endoparasites because the change from a free living existence to ectoparasitism does not appear to be so difficult as to what we see in endoparasidsm. The relationships between ectoparasitism and endoparasitism furnish much material for speculation in the evolution of the parasites.
Host-Specificity and Survival Value:
A parasite (essentially the endoparasites) needs a definite condition in the host. In almost all cases, the physiological demands of the parasites remain more plastic than its environment. But there is always a limit to such plasticity, as a result most of the parasitic species become host-specific.
Host-specificity arises because a particular parasite is adapted to take specific type of food especially the protein which produces specific antigen and antibody reaction. The degree of host-specificity bears a close parallelism with the evolution of parasitism. The more host-specific a parasite is, more divergent it appears to be from its original phylogenetic stem.
Reproductive Faculty and Survival Value of Parasitism:
The truth of evolutionary progression is that the mutation offers the species new variations for the Natural Selection to work with. The main source of mutation lies in the process of reproduction. In parasites the possibilities of mutation are lowered because most of the parasites are hermaphroditic and their genotypes are practically devoid of heterozygocity.
But this is compensated by the possession of specialised methods such as the precautionary measures in egg lying taken by the parasites to have good chance of survival. Prolific rate of egg production gives ample scope for mutation.
The phenomenon may be illustrated with the examples of Ascaris which produces about 200,000 fertilized eggs in a day. Fish tapeworm lays about 36,000 to 1,000,000 eggs per day. It is known that the mutation is the failure of faithfully copying the genes.
So when there is prolific rate of egg production, the rate of frequency of mutation increases. By this way of rapid duplication, numerous mutations are produced which provide the raw materials for Natural Selection to work with to reach the evolutionary goal.
While any mutant gene arising in the host may be favoured by Natural Selection as far as the elimination of the parasites is concerned by creating a different internal environment for that species of the parasite.
The mutations in the parasite also may be favoured by Natural Selection to adjust the parasite in the altered environment or establish it in a new species of host, the sum total effect of this battle between the host and the parasite is the possibility for emergence of new species both in the parasite and the host.
Amongst the possible modes of living in nature, the parasitic existence, although regarded to be a case of degeneration, is certainly a most specialised type of living and has great survival value. Now question arises how the parasites became parasitic. When a parasite from the generalised environment comes under the shadow of parasitism, it becomes confined in a specialised environment and becomes isolated.
In the realm of parasitism Natural Selection works in a most contradictory way. The survival of a parasite depends on the host-parasite relationship. Parasites cannot to be too much parasitic. The host must live for providing house for the parasites to live. Natural Selection works in two ways—in one hand it insists the parasites to be parasitic and on the other hand it checks them not to become too much parasitic.