In this article we will discuss about Calotes:- 1. Subject Matter of Calotes 2. Structure of Calotes 3. Sense Organs 4. Ecdysis.
Subject Matter of Calotes:
Example of the Class Reptilia — Calotes:
Calotes versicolor is a common Indian garden lizard. It is often called a blood-sucker but in reality, it does not suck blood. The colour of the body is changeable and in males the head is often blushed red.
Systematic position According to J. Z. Young, 1981:
Subphylum Vertebrata (= Craniata)
Calotes versicolor (Daudin).
Habit and Habitat:
Calotes passes much of its time lying on boughs and twigs. It is often seen running swiftly on the ground. It can swim, if necessary. Calotes lays its eggs in holes in the ground. It is widely distributed in India and Southern China.
Structure of Calotes:
The body of Calotes is divisible into three regions: head, trunk and tail (Fig. 8.2A). A small narrow neck joins the head with the trunk. The head is more or less triangular in appearance. The snout is short, pointed and bears a pair of apertures, called external nares. The eyes are provided with movable nictitating membrane. The mouth is a transverse aperture.
It is terminal in position and lips are absent. The tympanum is situated behind the eyes. The trunk is elongated, sub-cylindrical and bears two pairs of appendages, the fore-limbs and hind-limbs. Each forelimb is divisible into brachium, anti-brachium and manus. Likewise the hind limb is divisible into femur, crus and pes.
The limbs end in digits and the digits are provided with sharply pointed claws. The cloacal aperture or vent is a transverse opening and is situated in the posteroventral part of the trunk and at the base of the tail. A large cloacal plate is situated in front of the cloacal aperture.
The tail is long, slender and tapering. It measures twice as long as the head and the body. Males measure 120-140 mm from snout to vent with a tail of 300-500 mm. The body length of females is less by 15-20 mm.
The colour of the body is brown or greyish on the dorsal surface and dirty whitish ventrally. The dorsal surface may be uniform in colour or may have dark-brown transverse spots or bars. Dark streaks radiate from the eye region. Young females often show two light yellow dorsolateral stripes. Fully grown males have greenish tinge. The tail exhibits alternate dark and light annuli.
Calotes, like many lizards such as chameleons, possesses a peculiar power of changing the colour of skin. Change in colour is due to the contraction or expansion of chromatophores (pigment cells) which are possibly under nervous control. The colour is a protective device and serves in protecting the animal from the enemies.
The skin of Calotes is highly modified for living on land. To safeguard the animal against the frictional contact with the dry ground as well as to prevent desiccation in dry air, the epidermis becomes greatly elaborated than the dermis. The skin glands, so common in amphibians, are almost absent. The only skin glands recorded in Calotes are the femoral glands in males (Fig. 8.3).
These are present along the posteroventral margin of the thigh. These glands become functional during the reproductive period. The absence of skin glands and acquisition of scales over the body are evolutionary achievements for living on dry land.
The skin of Calotes develops first as a simple cuboidal ectoderm. This ectoderm then differentiates into an outer periderm and an inner basal germinative layer. The epidermis becomes stratified due to addition of outer layers and the scales appear subsequently.
The epidermis is a compound layer which is composed of an inner layer of columnar cells, called Stratum germinativum (or Malpigian layer) which is arranged on a basement membrane.
The outermost layer of epidermis is represented by a dead startum corneum. Between the stratum corneum and the stratum germinativum lies the transitional layer. The stratum corneum becomes thickened over the scales, while between the scales this layer becomes thinner.
Calotes has definitive exoskeletal structures outside the body and well-developed framework of bony endoskeleton.
Being a terrestrial animal, Calotes possesses well-developed exoskeletal structures. The body is covered by a non-conducting coat of keratin scales.
The skin of Calotes, like other reptiles, is covered by epidermal scales. Each scale covers an epidermal and dermal scale elevation. The scales form a continuous cover over the entire body of Calotes but become thinner in the grooves between the scales.
Closer examination reveals the presence of two types of scales:
(i) Larger scales and
(ii) Smaller scales lying between the larger scales.
Sensory structures in the form of sensory bristle (Fig. 8.4) are present on some large as well as small scales. These sensory structures are called prototrichs (plural of protothrix). The prototrichs of reptiles have been claimed by many to be precursors of the mammalian hair. It is quite reasonable that the small scales margining the larger ones are the forerunners of feathers and hairs.
The scales are epidermal in origin and are arranged in an imbricate fashion. The scales on the ventral surface of the body are smaller in size than those on the dorsal side. The scales develop from the stratum germinativum (or Malpighian layer) of the epidermis.
These are plate-like structures, each is supported by a bony plate or ossicle or osteoderm (see Fig. 8.2B). The osteoderms are tied into the dermis by Sharpey’s fibres.
On the dorsal side, the postaxial scales are larger than the pre-axial scales. The scales on the head region are large and do not overlap but touch each other by their edges. These larger scales are called head shields.
A few scales situated on the mid-dorsal line and on the border line of neck have become modified into large and pointed structures. These are capable of movement and are usually called frills. Similar structures are also present on the posterolateral border of the head. The scale cover is periodically shed by a process, called ecdysis.
The digital tips of Calotes are provided with sharp claws. Each claw is made up of dorsal and ventral scale-like horny plates. These plates are so placed that they converge at the end of the digit to make the tip pointed.
The dorsal plate is called unguis. It is convex and rounded at tip and lateral sides. The ventral plate is called sub unguis. The sub- unguis is flattened (see Fig. 8.2C). The plates are derived from the Malpighian layer of the epidermis. The unguis is well-developed than sub unguis.
The hard skeletal parts present inside the body (endoskeleton) are grouped into axial and appendicular divisions. The axial skeleton includes the skull and vertebral column while appendicular skeleton includes the girdles and limb bones.
The skull of Calotes is well-ossified but cartilaginous elements persist in the region of nares and inter-orbital septum. The brain box (or cranium) is comparatively small and is covered by investing bones.
The roof of the skull is made up of a pair of parietals which are fused and bear a median parietal foramen, a pair of frontals and two nasal bones (Fig. 8.5A). The premaxillae are situated anterior to and between the nasals while the sickle-shaped squamosal’s are attached to the posterolateral sides of the parientals by sutures.
On the roof of the orbit there are many small bones, of which the anterior and posterior ones are known as prefrontal and postfrontal respectively. The floor of the skull is composed of a flat and large basal bone formed by the fusion of occipital and sphenoidal elements. The basal bone continues forward as a slender bar and is called Para sphenoid (Fig. 8.5B).
From the basal bone arise a pair of stout basipterygoid processes. These processes articulate with the pterygoid. The pterygoid forms a bridge between the quadrate (posteriorly) and palatine (anteriorly). From the junction of pterygoid and palatine arises a stout os transversum or trans palatine process which extends to join the maxilla.
The vomer is small, anterior to palatine and articulates with premaxilla and maxilla. The premaxillae and maxillae are furnished with small teeth.
The posterior part of the skull is made up of large exoccipitals which are continuous with the horizontal prootic processes. The foramen magnum is an aperture situated between the occipitals. There is a single occipital condyle. The lateral wall of the cranium is mainly formed by a paired prootics.
The epipterygoid (or columella) is extended from the parotic to the pterygoid. The jugal bones border the lateral and posterior limits of the orbit (Fig. 8.5C). The quadrato jugal is not ossified. The lower jaw is composed of two similar halves or rami.
Each ramus, in addition to the slender persistent Meckel’s cartilage, consists of six pieces of investing bones which fuse with each other. The names of the six bones are: articular, angular, supra-angular, coronoid, splenial and dentary. Of the bones, the dentary has mandibular teeth (Fig. 8.5D).
The two rami are sutured to one another in front. The lower jaw is articulated to the cranium through the quadrate which is movable and such attachment of lower jaw is streptostylic in nature. The lower jaw bears teeth.
The vertebral column consists of many vertebrae.
The vertebral column is divisible into the following regions:
(a) Cervical region—composed of 8-10 vertebrae,
(b) Thoracolumbar region—made up of 22 vertebrae,
(c) Sacral region—containing only two vertebrae and
(d) Caudal region— consisting of about 20 vertebrae. All vertebrae, excepting the first and the last, are strongly procoelous.
The first cervical vertebra is known as atlas (Fig. 8.6A). It is ring-like having a single articular facet. The centrum is absent. The neural arch is incomplete dorsally and transverse processes are ill- developed. The second cervical vertebra is called axis (Fig. 8.6C). It bears a small peg-like process, called odontoid process at its anterior face.
The odontoid process actually represents the centrum of the atlas. The mid-ventral line of the axis is provided with a ridge, called hypapophysis. The rest of the cervical vertebrate are typical but bear cervical ribs. The atlas and axis do not bear ribs.
The thoracic vertebrae are typical in nature (Fig. 8.6B, D). The hypapophysis is ill-developed or absent. All the vertebrae bear thoracic ribs. Only five anterior thoracic ribs reach the sternum.
The two large sacral vertebrae bear large and expanded ribs.
The ventral surface of the caudal vertebrae is provided with Y- shaped chevron bones.
The sternum (or breast bone) is a rhomboidal plate-like structure and ventral in position. The sternum is supported by a characteristic T-shaped interclavicle and it bears an aperture, called sternal aperture, at its middle (see Fig. 8.7A). the sternum bears ribs. The plate-like sternum protects the internal organs.
The floor of the buccal cavity is supported by a cartilaginous structure, called the hyoid apparatus (Fig. 8.6E). It also supports the tongue. The hyoid apparatus consists of basihyal, anterior cornua, middle cornua and posterior cornua. The basihyal constitutes the body proper of the hyoid apparatus. It is a median cartilaginous rod and directed anteroposteriorly to form the osentoglossus.
The anterior cornua are elongated cartilaginous rods and arise anterolateral from the basihyal. These rods curve round the oesophagus and terminate near the ventral side of the auditory capsule. The anterior cornua represent the hyoidean arch.
The middle cornua are ossified at their proximal ends and represent the vestiges of the first branchial arches. The posterior cornua arise from the posterolateral edge of the basihyal. They extend backwards and outwards. Each cartilaginous rod is composed of two segments.
iii. Appendicular Skeleton:
The skeleton of the paired limbs and girdles constitutes the appendicular skeleton.
The pectoral girdle is hoof-shaped and composed of two symmetrical halves. It is situated in the thoracic region of the body being attached to the vertebral column by means of muscles and ligaments. Each half of the girdle is made up of a partially cartilaginous scapula. The scapula is dorsolaterally extended (Fig. 8.7A).
The suprascapula is a bony plate-like structure and is attached to the scapula. It is ventrolateral in position. The coracoid is also plate-like and is made up of cartilaginous epicoracoid and precoracoid. The glenoid cavity is formed by the scapula and coracoid. The clavicle is a narrow-curved bone and forms a bridge between inter-clavicular junction and scapula.
The two halves of the girdle are attached to a rhomboidal and plate-like sternum which is ventral in position. The sternum is supported by a T-shaped episternum (or interclavicle). The sternum bears ribs and a centrally placed sternal foramen through which the sternal artery passes. The T- shaped interclavicle, sternal ribs and sternal foramen are diagnostic features of the reptilian pectoral girdle.
The pelvic girdle is also made up of two equal halves. Each half is called os innominatum which is composed of pubis, ischium and ilium (Fig. 8.7B). These three bones are attached in a tri-radiate fashion. The two halves of the girdle are attached with each other through the intermediation of pubis and ischium. Hence the joining is called the ischiopubic symphysis.
The pubis bears an epipubic cartilage at its tip and a pubic prominence as a projection at the other tip. The articulation facet, acetabulum, is situated close to the pubic prominence. Between the two ischia there lies the hypoischiatic cartilage. The obturator foramen is incomplete.
The forelimb consists of the following long bones arranged in order from proximal to distal end (Fig. 8.7C). Humerus is a slender and slightly curved bone which bears swellings (or condyles) at the proximal end. This swelling is called head of humerus. The distal end bears a pulley-like trochlea. Spherical epiphysis is associated with both ends. The deltoid ridge is absent.
The anti-brachium is supported by radius and ulna. These two bones are narrow, elongated and are distinct from each other. The radius is pre-axial in position and its distal end is produced into a process, called styloid process while the anterior terminal end of ulna is produced into a short process, called olecranon process.
The radius and ulna are articulated with the humerus through their anterior concavities. In manus, there are ten pieces of carpal bones. The carpal bones are cubical in shape and are arranged in two rows.
The proximal row consists of a large central piece, called centrale, one piece each beneath radius and ulna are called radiate and ulnare respectively while the third and fourth pieces are fused together to form the intermediate.
The distal row of carpals does not show structural variation. An extra piece of bone developed pre-axially between ulna and ulnare is present. This bone is called pisiform. This is a sesamoid bone (or tendon bone) and its presence is very unusual. Five narrow but cylindrical metacarpals form the palm. The five phalanges are provided with claws and the bones in phalanges number 2, 3, 4, 5 and 3 from 1st to the 5th.
The hind limb consists of a stout and elongated femur which is provided with epiphysis at both the ends. The anterior end of the femur is round while posterior end bears two ball-shaped condyles. Above the outer condyle, there is an elevated portion which gives attachment to the next pieces of bone. The anterior side of femur is provided with a rough-surfaced trochanter to which muscles remain attached.
The shank is composed of tibia and fibula (Fig. 7.7D). The tibia is pre-axial in position. It is straight and stouter than fibula. The tibia bears a cnemial crest at its anterior surface and a concavity at its posterior part for articulation. The fibula is slender and bears a single concavity through which it articulates with the elevated portion of femur.
The distal ends of both tibia and fibula are flat. In tarsal region, the bones are fused. A single large piece, called tibiofibulare and two or three bones on the distal region are only identifiable. The foot is supported by five cylindrical and elongated metatarsal bones. Of these bones, two articulate with tibiofibula and the rest three with the distal tarsal bones. The digits are similar in structure to that of the fore- limbs.
Pineal Eye in Reptiles:
Majority of the reptiles possess a parietal organ on the dorsal side of the diencephalon and lying in the parietal foramen or just below it. The parietal organ develops as an outgrowth from the anterior part of the pineal body or directly from the cerebral roof.
The organ often remains in close association with the skin. The parietal organ has a vesicular appearance and the walls are composed of simple epithelium. This organ, in many cases, present peculiar modifications resembling the structure of a vertebrate eye.
In some cases, as in Sphenodon, the thickened dorsal wall imitates a lens and the rest of the wall recalls that of a retina. In many reptiles, a cord of tissue connects the parietal organ with the pineal body.
But in Calotes, Sphenodon and many others the parietal organ stays quite separate from the brain (Fig. 8.15). Because of the eyelike structure and on account of its connection with the roof of diencephalon or with the pineal body in many reptiles, the parietal organ is often referred to as pineal or third eye.
Considerable controversies exist on the functional significance of the ‘pineal eye’. The skin over the top of the parietal organ, in some reptiles, becomes de-pigmented and assumes a cornea-like appearance. Despite eye-like appearance, the parietal organ has no trace of any visual function.
The contention that this organ is a vestige of once functional median, eye is not supported by histological evidences. Further research on this line is necessary to ascertain the significance of this organ.
Sense Organs of Calotes:
The sense organs are highly developed in calotes.
The olfactory organs for smell in Calotes are the nasal chambers. The nasal chamber or passage extends between the external and internal nares. The anterior part of the nasal chamber is devoid of sensory cells while the posterior part bears many spindle-shaped cells connected to the twig of the olfactory nerve.
Highly segmented sensory epithelial cells cover the spindle shaped cells. To enlarge the surface of exposure, the epithelium of nasal wall is extended into a single shelf like concha. Calotes has single unrolled concha one on either side.
Organ of sight:
The sense organ for sight consists of a pair of eyes which are lateral in position. The eyes are provided with eyelids and glands. The upper eyelid is small and less movable and the lower eyelid is large and more movable. The third eyelid or nictitating membrane is transparent. Harderian glands supply the third eyelid, while lacrimal glands make their first appearance in reptiles.
The wall of the globular eye is made up of several layers (Fig. 8.18A). The inner-most layer bounding the posterior part of the eye is made up of retina. It is supplied with fine branches of nerves from the optic nerve. A vascular pigmented cushion-like projection arises from the blind spot region of retina. This projection is called Pecten or ectodermal conus.
The layer outside the retina is called choroid which is made up of an inner pigmented layer and an outer vascular layer. Outside the choroid is situated the sclerotic layer which is protective in nature. The front of the eye is occupied by lens which is a clear ball derived from the skin (ectoderm). The iris is a pigmented and muscular ring in front of the lens.
The iris is a prolongation of the choroid. The aperture in this iris in front of the lens is called pupil. The front part of the eye is protected by cornea which is continuous with the sclera. The cornea is protected by conjunctiva, which is epidermal in nature. The lymph space between cornea and iris remains occupied by aqueous humour and the inner space between lens and retina remains filled up with jelly-like vitreous humour.
Organ of Jacobson:
An accessory structure, called organ of Jacobson or vomeronasal organ, is associated with the olfactory organ of Calotes (Fig. 8.18B). It is a cavity derived embryonically from the olfactory cavity, lined with sensory epithelium and innervated by the vomeronasal division of the olfactory and also by the trigeminal nerves.
It is a small sac-like structure with heavily pigmented walls supported by cartilages. An organ of Jacobson is present below each nasal cavity.
It is completely separated from the nasal cavity in adult. Paired ducts communicate with the mouth cavity. It acts as a chemo- receptor and it is highly developed in lizards and snakes. This sense organ appreciates scent particles introduced into it by its tongue tips. It has also been experimentally established that this organ in some reptiles helps in trailing prey and locating the opposite sex.
Auditory sense organ:
This sense organ comprises of ears which perform two-fold functions—hearing and maintenance of equilibrium.
The ear of Calotes is made up of two parts:
(a) The middle ear and
(b) The internal ear.
External ear is absent.
It is tubular cavity with its outer opening being covered by the tympanic membrane situated behind the eye. The membrane is slightly depressed and is perforated by very minute apertures. The inner end of the tube is covered by a very thin membrane. A bony rod like structure called columella auris is situated inside the tube of the middle ear.
The role of this structure is to transmit the sound waves. The middle ear is communicated to the buccal cavity by a narrow but long tube called Eustachian tube. The middle ear communicates with the internal ear through a circular structure called fenestra ovalis.
The internal ear is situated in a bony capsule. The bony capsule has two fenestrae—fenestra ovalis and fenestra rotunda. The columella auris is fixed internally into the membrane which covers the fenestra ovalis. The space between the capsule and internal ear is filled in with perilymph. The internal ear is demarked into an upper utriculus which is tubular and slightly bent and a lower sacculus which is sac-like.
Lagena or rudimentary cochlea is attached to the ventromedian side of the sacculus (Fig. 8.18C). The lagena is of simple type and contains the papilla basilaris, a special organ of hearing. From the utriculus arises a tubular structure called ductus endolymphaticus which opens beneath the duramater of brain.
There are three semicircular canals attached to the utriculus by both ends. Each semicircular canal is swollen at one end. The swollen part is called ampulla. The ampullae of anterior semicircular canal and horizontal semicircular canal are situated side by side while those of posterior semicircular canal and horizontal canal are situated at a distance.
Minute masses of CaCO3 present inside the ampullae help in maintaining balance. These pieces of CaCO3 are called statoliths or autoliths. The sacculus, utriculus and the three semicircular canals remain filled with a liquid, called endolymph.
Ecdysis in Calotes:
Ecdysis is nothing but periodical peeling off of skin. Ecdysis occurs in Calotes and thus scales lodged in skin are shed. It is assumed that some amount of nitrogenous elements is thrown out during the process of ecdysis. So ecdysis is also a process of excretion.