The following points highlight the seven main types of system existing in fishes. The types are: 1. Fin System 2. Digestive System 3. Respiratory System 4. Circulatory System 5. Nervous System 6. Excretory System 7. Reproductive System.
Type # 1. Fin System:
The fins constitute the major propulsive organs in fishes. These are either folds of skin or projections from the body surface. The fins are supported by fin-rays. These supporting rays may be bony, cartilaginous, fibrous or horny.
There are mainly two types of fins in fishes:
(a) Unpaired or median fins and
(b) Paired fins.
The median fins include the dorsal, caudal and anal. The paired fins are the pectorals and the pelvics. These paired fins correspond to the paired appendages of land vertebrates. A great variety of fins is observed in fishes. The diversity in the fin system in fishes is due to their adaptive responsiveness.
Origin of fins:
The median or unpaired fins in fishes are held to be originated from a continuous fold of tissue. This fold extends from the posteriorly around the tail and forward up to the anus. This fold is supported by series of parallel cartilaginous rods. In course of development each supporting rod divides into a lower piece, i.e., basal, embedded in the body wall and an upper piece lying in the fin-fold, i.e., radial.
From such a continuous fin-fold, the dorsal, caudal and anal fins have been evolved by restriction of the radials at certain areas and the progressive degeneration of the fold between them. Many Ichthyologists are convinced about the derivation of the unpaired fins from the continuous fin-fold, but controversy exists as regards the origin of the paired fins in the phylogenetic development of the fishes.
Two conventional theories are:
(A) Gill-arch theory and
(B) Fin-fold theory.
However, recently many scientists have discarded the fin-fold theory and have tried to establish the idea that fins arose from spines.
The idea of the origin of paired fins from the gill-arches was first advanced by Cagenbaur. According to him, the paired fins are the modified gill-structures and the girdles represent the gill-arches. The position of the pelvic fins can be explained on the assumption that some of the posterior gill-arches have been shifted posteriorly.
This idea finds no support from embryological, morphological and palaeontological studies.
Lateral Fin-fold Theory:
This theory was first proposed by Balfour and Thacher and later on supported by many eminent Zoologists, like Wiedershiem, Parker, Goodrich and many others. According to this theory, the paired fins have originated from a paired lateral fin-folds running down each side of the body behind the gill-openings up to the end of the tail (Fig. 6.72).
These lateral fin-folds are separated at the anterior part, but become fused posteriorly as a median ventral fin-fold. There is also a continuous median dorsal fin- fold.
The condition of the lateral fin-folds in the ancestral forms was presumably like the meta- pleural folds of Branchiostoma. Existence of such lateral fin-folds is furnished by an archaic fossil vertebrate, Famoytius kerwoodi discovered from the upper Silurian and also from the study of some fossil cyclostomes and some primitive fishes.
There are many palaeontological and embryological evidences as under:
(a) Although the structure of the paired fins varies greatly in different fishes, the mode of development is strikingly similar. The embryological stages of fin formation give support to the presence of continuous fin-fold.
(b) The condition of the paired fins in an extinct shark, Cladoselache added more weight to this idea. The pectoral and pelvic fins in this form are very wide and lack posterior and anterior notches from the body wall. Presence of such constrictions is a characteristic feature of other fishes.
The fins are supported by parallel fishes. The fins are supported by parallel fishes. The fins are supported by parallel arrangement of simple cartilaginous rods. Such an arrangement of supporting rods and lack of notches are suggestive of the origin of the fins from the continuous fin-folds.
(c) In lower Devonian placoderms (Acanthodians), a series of spines extends ventrally from behind the head to the anal fin. This also suggests the presence of continuous fin-fold. The idea of the fin-fold origin of the paired and unpaired fins in fishes is supported by anatomical, embryological and palaeontological evidences.
This theory was accepted by most of the workers in this-field of research and the gill-arch theory of Gegenbaur has become a theory of historical importance.
But recently evidences are put forward against the fin-fold theory. As regards the question of meta-pleural folds in Branchiostoma, it can be suggested that this point appears baseless as Amphioxus is now removed from the direct line of vertebrate evolution. Indeed in the oldest known ostracoderms the fins were short-based and not broad-based.
Gregory and Raven (1941) showed that the broad-based fins of Cladoselache were highly specialised, not primitive in nature and they are actually analogous to those of rays. Recent workers claim that the so-called ‘several pairs of fins’ in acanthodians indicate nothing but a special kind of multiplication of the defensive spines, which may often support membranous structures.
Furthermore, no true primitive fossil fish with completely continuous fins is still known. This finding forces the recent workers to discard the fin-fold theory. They suggest that the fins appeared probably in connection with some paired and unpaired median spines, as found in many ostracoderms. In course of evolution membranous structures appeared between these spines and the adjacent body-wall.
As stated above the unpaired fins include the dorsals, caudal and anal.
Dorsal and Anal Fins:
These unpaired fins show great variation in structure and disposition. Amongst the Elasmobranchii, the primitive living forms like Chlamydoselachus, Heptranchias and Hexanchus possess Single dorsal fin, while in the rest of the forms the dorsal fins are two in number. The fins are supported by many horny fin-rays (ceratotrichia) located beyond the cartilaginous radials.
Both the ceratotrichia and the radials remain completely covered by the integument. But in the bony fishes, the radials lie in the muscles and the basals within the body. In higher teleosts, the radials become reduced to bony or cartilaginous nodules.
The nodules are situated within the muscles of the body and are attached with the fin-rays (lepidotrichia). The fin-rays are actually the modified scales. Usually the number of the fin-rays corresponds directly to the number of the radials, but in the Dipnoans and Sturgeons, the fin-rays exceed the number of the radials.
In many actinopterygians, the dorsal fin is preceded by many spines. Presence of a single dorsal fin is a primitive feature amongst the actinopterygians. This condition is observed in fossil forms and to some extent in creatures like Lepisosteus and Acipenser. In higher forms there are two dorsal fins.
The crossopterygians possess two dorsals, of which the posterior one is larger excepting in Latimeria where the condition is just the reverse, i.e., the anterior one is larger. In Polypteus, the dorsal fin is divided into a number of fin-lets, each having one stout spine supporting a membranous flap.
The modifications of the unpaired fins are related with their particular role in propulsion. In the sharks, the dorsal fins are well- developed and act as stabilizer, but in the rays the dorsal fins are reduced because they are adapted to live at the bottom of the sea. In Sting-rays and Eagle-rays, the dorsal fins are totally absent.
In Bull-headed sharks, both the dorsals are preceded by a strong sharply pointed spine. The spines are defensive organs and are also associated with the poison glands as seen in Squalus (spiny dog-fish).
In bony fishes the dorsal fin shows extensive variations, specially in shape, size and position. It is present in almost all the forms excepting Electrophorus (Electric eel), where it is either lost or reduced to a mere filament-like structure.
In primitive bony fishes, the fins are supported by flexible rays, and in the actinopterygians the rays have become converted into stiff spines. Man eel, Acanthenchelys, the dorsal fin is supported by both soft rays and stiff spines.
The development of the spines in the unpaired fins in many fishes added a new function of defence in addition to their normal role of propulsion. The spines exhibit great variations amongst the fishes. In many fishes, notably Trachinus and Syrianceia, the spines supporting the dorsal fin are connected with the poison glands.
Extreme modification is observed in the sucker fishes (Echeneis, Remora), where the spinous dorsal fin becomes transformed into an oval adhesive disc over the head. In Angler- fishes, the dorsal fin is highly modified. The first ray of this spinous fin is located on the snout and transforms into a flexible filament with a membranous appendage at its tip.
In deep-sea Angler fishes inhabiting the totally dark realm, a luminous bulb is present on this appendage. The luminous bulb varies in shape and size and produces light to allure and attract small fishes. In Lasiognathus, this appendage is composed of a stout rod-like basal part and a slender filament. Besides the luminous bulb, a series of non-functional hooks is present.
The anal fin, like that of other unpaired fins, also shows variation in shape and size in different fishes. In fishes, where this fin acts as a locomotor organ, the anal fin is greatly elongated. It is usually supported by soft rays. In some forms, a few anterior rays become transformed into spines. The anal fin is mostly single, but in Cadus and some other related forms the anal fin is divided into two parts.
The anal fin in the males of the South American Cyprinodonts becomes modified into complicated intermittent organs. The third to fifth fin-rays are enlarged and contain either a groove or closed tube into which the reproductive ducts open.
Of the unpaired fins, the caudal fin plays the most important role in forward propulsion during swimming. The caudal fin is highly developed in most fishes excepting Hippocampus and some of the eels under the order Anguilliformes.
In Hippocampus (Seahorse) the tail in the latter forms is produced into a long whip-like tapering structure. The caudal fin in the rest of the fishes exhibits a good deal of variation (Fig. 6.73). Three major types of caudal fins are encountered in different fishes.
Protocercal or Diphycercal:
This type of caudal fin is regarded to be the most primitive type. The vertebral column extends up to the tip of the tail and divides the caudal fin into two equal halves. The dorsal half is called epichordal lobe and the ventral one is known as hypochordal lobe. The epichordal and the hypochordal parts of the caudal fin are equal in size and symmetrical (Fig. 6.73 A, B, C).
All fishes pass through this stage during development. In the living dipnoans, the caudal fin is of protocercal type. In Latimeria, this condition is slightly modified. It is protocercal with a sharply marked median lobe. Presence of protocercal tail in some highly developed fishes is possibly due to secondary modification.
In this case, the vertebral column bends upwards. As a consequence, the caudal fin is divided into two unequal halves. The vertebral column is bent upwards and continues almost up to the tip of the fin. The epichordal part is greatly reduced while the hopochordal part is greatly reduced while the hypochordal lobe is specially enlarged to make the caudal fin asymmetrical both internally as well as externally (Fig. 6.73D, J).
This type of caudal fin is found in elasmobranch, extinct crossopterygian and primitive actinoptrygians. In most elasmobranchs, the caudal fin is usually heterocercal but in Chimaera and Chlamydoselachus the caudal fin is of isocercal type (Fig. 6.73E).
This type of caudal fin is the characteristic of the higher bony fishes. The fin is symmetrical externally but internally it is asymmetrical (Fig. 6.73C). The posterior end of the vertebral column is turned upwards and becomes greatly reduced.
The tip of the vertebral column does not reach the posterior limit of the fin. There is no apparent dorsal lobe but the ventral lobe is greatly enlarged and divided into two equal superficial lobes. Most of the teleosts retain the typical homocercal condition, but in some forms this condition is slightly modified.
The types of modifications encountered are as follows:
(a) In Cod and Tuna, the homocercal condition is modified (Fig. 6.73H,I). The upward terminal portion of the vertebral column is withdrawn.
(b) In some deep-sea fishes (Anguilliformes, Notopteridae, Gymnarchidae, Macruridae), the upturned tip of the vertebral column becomes elongated and straightened out. The fin is actually the marginal extension above and below the elongated vertebral tip.
The fin is supported by rays. Such a type of caudal fin is called isocercal. Developmentally, the isocercal tail is formed by the reduction of size of the hypochordal lobe and elongation of the dorsal and anal fins, so that a continuous fin-fold is re-established.
(c) In Fieraspis and Orthagoriscus, the caudal fin completely disappears with the truncation of the vertebral column. Such a type of fin on a rounded caudal lobe is regarded as gephyrocercal fin (Fig. 6.73K).
The protocercal or diphycercal type of caudal fin is considered as the most primitive type, the heterocercal as the intermediate stage and the homocercal condition represents the advanced stage. In many teleosts, the caudal fin starts as dihycercal, then becomes heterocercal and finally assumes the homocercal condition.
The transition of three types of caudal fin in the developmental history of fishes is significant from the phylogenetic point of view.
That the homocercal caudal fin is derived from the heterocercal type is evident from the transitional types amongst the extinct and living bony fishes. In primitive forms like Acipenser and Polyodon, the caudal fin is strongly heterocercal, while in Amia and Lepisosteus, the caudal fin exhibits an intermediate condition between the heterocercal and homocercal type.
The vertebral column is shortened and retracted at the base of the caudal fin.
The fin has retained the reduced upturned fleshy epichordal lobe, but the hypochordal lobe is greatly enlarged to form only the anteroventral lobe. This condition is called either the heterocercal or abbreviated homocercal type. Most of the early fishes possess heterocercal type of caudal fin.
From the heterocercal condition the homocercal caudal fin is evolved by gradual reduction of the upward (epichordal) fleshy lobe. The homocercal condition is transformed into isocercal or abbreviated homocercal or gephyrocercal types of caudal fin.
The paired fins, the pectorals and pelvics, correspond to the forelimbs and the hind limbs respectively of the tetrapods. The pectoral fins are usually situated just posterior to the gill-opening. The position of the ectoral fins varies slightly in different fishes. But the position of the pelvic fins varies considerably in different forms.
Both these paired fins are supported by dermotrichia (fin-rays) and somactids (radialia). In Cladoselache, a primitive extinct shark, the somactids are arranged parallel to one another and are termed as orthostichous type (Fig. 6.74A). The rachiostichous type of fin skeleton i.e., the somactids are represented by a jointed basal piece articulating with the girdle and provided with a posterior postaxial and anterior pre-axial radials.
The radials are attached to the basal piece is observed in Ceratodus. Such a leaf-like structure of the fin is called archipterygium. In many fishes, particularly in elasmobranchs, the fin-skeleton assumes a fan-like form and is described as rhipidoschous type. The question of primitiveness of the archipterygium type of fin as advanced by Gegenbaur has been challenged by Balfour.
According to him the fin skeleton found in Cladoselache with parallel arrangement of somactids appears to be more convincing. The parallel somactids supporting the paired fins are possibly originated from the local specialisation of once parallel somactids of the continuous fin-fold.
The pectoral fins in the elasmobranchs are highly developed and are larger in size in comparison to that of bony fishes. In skates and rays, the pectoral fins become enormously developed on the sides of the body and head.
These lobe-like pectoral fins act as the principal locomotor organs. In bony fishes, the pectoral fins are paddle-like and small. In many bony fishes the outer margin of the fin is provided with a stout spine. In some Catfishes, the pectoral spine is associated with a basal poison gland. The spines are defensive organs and in Doras and Clarias the pectoral spines also help in progression on land.
The pectoral fins are present in almost all the fishes except some of the Eels and Pipefishes where the pectorals are wanting. The pectoral fins are always longer in speedy fishes but in the fishes of slow movement these are broad and round. Neoceratodus (Fig. 6.74C) possesses peculiar lobate leaf-like pectorals, while in Lepidosiren and Protopterus, the pectorals become extremely elongated and filamentous.
In Exocoetus (flying fish) the pectoral fins have been extremely enlarged into the ‘wings’ and help to make short aerial excursion. In Pantodon another flying fish of African rivers, the pectorals are greatly elongated and beat rapidly during flight. In Frog- fishes, Sea toads and Bat-fishes, the pectoral fins assume the appearance of ‘hands’ and help to crawl on the bottom of the sea.
The mud-skipper (Periophthalmus) makes a short excursion over marshy land and the pectoral fins are specialised for the purpose. The fin is membranous and is attached with a muscular stalk. In many fishes like Polynemus, Latris, some of the fin-rays supporting the pectoral fins are modified for sensory function. These fin-rays have drawn out into delicate filaments containing sense organs.
Pelvic fins. Like that of pectoral fins, the position and shape of the pelvic fins vary greatly in different fishes. The burrowing fishes usually lack the pelvic fins. In all the Eels and Pipe-fishes the pelvic fins are lacking. In many bony fishes under the family Cadidae (Cods), the pelvic fins are reduced to filaments.
In bony fishes, the pelvic fins may be abdominal (between the anal and pectorals) or thoracic (more or less below the pectorals) or jugular (in front of the pectorals) in position.
The conspicuous specialisation of the pelvic fins lies in their transformation into sucking disc for attachment with stones or any other object. Gastromyzon possesses a large sucker and both the paired fins participate in the formation.
In Cyclopterus (Lump-sucker) and Liparis (Sea-snail) similar sucking disc is present. Another remarkable modification of the pelvic fins is observed in the skates and rays where the distal part of the pelvic fin is modified into claspers (myxopterygia) in males.
Type # 2. Digestive System:
Fishes devour various types of food. Some of them feed exclusively on plant materials, others on small fishes and the rest are omnivorous or carnivorous. As a result of adaptation to different feeding habits, the digestive system becomes extremely modified. The structural plan of buccal cavity, pharynx and the rest of the gut in fishes has become modified in response to the feeding habits.
The alimentary canal regions are:
(iv) Intestine and
Histology of Gut:
The alimentary canal in fishes has four distinct histological layers—serosa, muscular layer, sub-mucosa and mucosa. The buccopharynx is lined by mucosa (consists of stratified epithelial cells), sub-mucosa (consists of loose connective tissue) and thin muscularis having scattered longitudinal muscle fibres.
The mucosa of the buccopharynx contains large club-shaped cells, flask-shaped mucous cells and a few taste-buds especially in the hinder region.
The oesophagus is characterised by having prominent mucous folds lined by columnar epithelium. Many mucous cells are present in the mucosa. The sub-mucosa is thinner and contains scattered longitudinal muscles arranged in bundles. The muscularis is composed predominantly by circular muscle fibres.
The serosa is thin. The stomach has prominent folds in the mucosa and the muscularis is very thick. Numerous simple tubular gastric glands are present which open into the lumen of the stomach. The intestine is thin-walled. The mucous membrane is folded to form villi. The villi are numerous in the proximal part of the intestine where they may fuse with each other.
The mucosa contains columnar epithelium, absorptive and mucous cells. The sub-mucosa is highly vascular. The muscularis consists of circular and longitudinal muscles. The serosa contains blood vessels. The rectum has reduced muscle fibres with short and broad mucosal folds.
The first part of the alimentary canal of the fish is designated as buccopharynx which performs two functions:
(a) Collection of food and conveying them to the oesophagus and
In the predatory and carnivorous fishes, viz., Wallagonia attu, Channa punctatus, Notopterus chitala (Fig. 6.75), Harpodon nehereus, the buccopharynx is armed with strong teeth.
In herbivorous forms as exemplified by labeo rohita, Cirrhinamrigala, the buccopharynx is divided into two portions: (1) an anterior respiratory region and (2) a narrow posterior masticatory region. The plankton feeders (Tenualosa, Hilsa ilisha, Gudusia chapra) may lack teeth in the buccopharynx.
The nasal cavities are quite separate from the oral cavity in fishes which lacks glands excepting the simple mucous glands opening into it. The tongue in fishes is just a fold developed from the floor of the mouth cavity. The sensory receptors may be present in tongue which lacks muscles. The tongue may be absent in rays.
The teeth are modified according to the purpose they are used. The location of teeth also varies greatly. Besides their normal location on the jaws, they may be present on tongue and on the hyoid arch. In most cases, dentition is of polyphyodont type, i.e., the fishes (particularly the sharks) do not keep the same set of teeth throughout life and there is constant replacement of the older functional teeth by younger ones.
The teeth are usually of acrodont (i.e., the teeth are linked basally with the jaws by fibrous membrane) and homodont types. The size and form of teeth exhibit great diversity in fishes. Simple conical teeth bearing longitudinal grooves and ridges are regarded as the most primitive condition. Such teeth are present in extinct Crossopterygians and in Latimeria.
Usually the teeth in fishes are conical projections, but may become modified into vertically flattened triangular plates as seen in sharks. In chimaeras and dipnoans, plate-like structures are formed by the fusion of several teeth for crushing molluscan shells. In majority of teleosts, the teeth are recurved to prevent the escape of slippery prey. The teeth may be large in size in deep-sea teleosts.
There is no distinct differentiation between oesophagus and stomach. According to Barrington, the stomach is a special portion of the oesophagus. Originally it was meant for the reception of large pieces of food and secondarily it became modified to produce acid and the enzyme—pepsin.
The significance of the division of the stomach into a descending cardiac part and an ascending pyloric part is unknown. Stomach of various shapes may occur in different fishes. In elasmobranchs, the stomach is usually J- shaped, but in teleosts the shape varies greatly.
In teleosts the stomach is usually V-shaped and the cardiac region is prolonged into a blind pouch. In deep-sea teleosts, it is highly distensible. In certain forms, e.g., Cyprinus, Labeo, Fundulus and Labrus the stomach is totally absent.
In cases where true stomach is absent, the anterior portion of intestine behind the oesophagus becomes swollen to form a sac called intestinal bulb or intestinal swelling. The intestinal bulb is an organ for storage of food.
Presence of intestinal bulb is the characteristic feature of Cyprinids. Stomach is absent in Dipnoi, Holocephali, Scomberesox, Xenentodon and Hippocampus. The intestinal bulb does not play any digestive role because gastric glands are absent.
The intestinal bulb has only absorptive and mucus-secreting cells in the mucosa. Absence of stomach in these fishes is not possibly related to their feeding habit. In many teleosts, Hilsa, Mugil, Gudusia the stomach becomes greatly thickened to form a gizzard-like structure for the trituration of food.
The length of intestine varies in different fishes. In carnivorous fishes, the intestine is generally short and straight while it is long and coiled in herbivorous forms. Intermediate condition is observed in omnivorous fishes. But in some carnivorous fishes the intestine is long and coiled. So it is difficult to generalize the contention that the length of the intestine is dependent on the nature of diet.
Das and Moitra (1956) advocated that the ratio of the length of gut and the length of fish is nearly constant. It is also claimed that the short gut is physiologically compensated by having extensive mucosal foldings. Presence of a spiral valve in the intestine of elasmobranchs is very common. Its arrangement may vary greatly in different forms.
In some cases, the valve is not spiral, but rolled in the form a scroll. But the spiral valve is usually absent in osteichthyes excepting a few forms like Polypterus. In many forms (Lepisosteus, Amia) it may be vestigial. Many fishes possess pyloric caeca, but the number varies greatly. In Somniosus, there is a pair of pyloric caeca.
In Polypterus there is only one pyloric caecum, but the number exceeds two hundred as seen in Mackerel. In some cases the pyloric caeca may be bounded together by connective tissues to form a compact mass. The intestinal caeca are lacking in stomach less fishes. Histologically these caeca resemble the intestine and possibly increase the absorptive area of the gut.
The rectum is distinguishable by having ileorectal valve at the junction of intestine and rectum in many fishes. In elasmobranchs rectum terminates into a cloaca. A rectal or digit form gland opens into the dorsal side of the rectum. But in most teleosts, anus opens separately to the outside. The rectal gland is absent in Actinopterygians, but seems to be present in Latimeria.
In fishes, the liver is a large bilobed organ. The gall-bladder is present in almost all forms excepting a few sharks. Elasmobranchs have a distinct pancreas, whereas in most teleosts, the endocrine part of the pancreas remains encapsulated and separated from the exocrine part.
Such isolated areas are designated as the principal islands. The pancreas remains in a diffused state in the coils of intestine and even inside the spleen. The exocrine cells are arranged round the blood capillaries or they may form pancreatic acini.
Type # 3. Respiratory System:
The fishes possess well-developed respiratory organs. The physiological process of respiration in different fishes is essentially similar to that of any higher vertebrate, the only difference in the respiratory process being the organs of respiration.
Fishes are the primary aquatic vertebrates. They utilise the oxygen which remains dissolved mostly in water. A few fishes have the power to breathe air. In fishes the respiratory organs are the gills.
Besides, some accessory respiratory structures for aerial respiration are encountered in some teleosts. In fishes, where aerial respiratory structures have evolved, the gills in them play a subsidiary role.
The gills constitute the efficient respiratory organs which are specially modified to utilise up to 80% of the oxygen dissolved in water that passes over the gills, whereas in man, the lungs are capable of absorbing about 25% of oxygen from the air drawn into the pulmonary cavities.
The efficiency of the gills as the respiratory organs in fishes depends upon two factors:
(a) The structural organisation of the gills and the nature of vascularization and
(b) The gills are bathed by the continuous flow of water so that fresh oxygen is always brought in immediate contact with the gills.
Development of Gills:
The larvae of some vertebrates possess gills that develop from the ectoderm and lie outside the body. These are called external gills. Another type of gills lie in the head region, called internal cells. Both types of gills occur in aquatic gnathostomes such as fishes and in the larval stage and adult neotenic amphibians.
The internal gills develop in the pharyngeal region. The side walls of the embryonic pharynx are lined by the endoderm. The paired pouches develop in the endodermal part by invagination and the pouches are called pharyngeal or gill-pouches or visceral pouches (Fig. 6.76). These pouches are six or more in number. The pouches then proceed to the outside through mesodermal layer.
At the opposite side of each pouch similar groove-like structure of the ectoderm develops by invagination. These groove like structures, called visceral grooves. The pouches and grooves are joined by the separation of the intervening thin partition, called closing plates. By perforation through the closing plates, slit-like structured develop called gill clefts or branchial clefts or gill-slits.
The external gill-slits are situated on the side of the anterior end of the head between the eyes and the pectoral fins and the slits on the side wall of the cavity of pharynx, called internal gill-slits.
The serially arranged pharyngeal or visceral pouches at the two sides are separated by visceral arches and arches contain arteries, called aortic arches. The pharyngeal or gill or visceral pouches are separated by inerbranchial septa which form the walls of the pouches. These septa bear gill-filaments (Fig. 6.76).
According to the structure and arrangement, the gills of the vertebrates are classified into 3 types:
a. Pouched Gills:
This type of gills is found in agnathans (Fig. 6.77). These are spherical or pouch-like muscular gill chambers situated at the side of the pharyngeal region. In Lampreys there are 7 pairs of branchial or gill pouches which remain on each side of the body.
The pouches open directly into the respiratory tube and have no direct connection with the enteric canal. The gill pouches open to the outside by external pores. One end of the respiratory type opens to the buccal cavity and the other end is blindly ended.
The inner wall of the gill pouch is folded 10 form numerous gill lamellae and the outer wall is highly muscular. The gill pouches are separated from each other by inter-branchial septa. In Hagfishes the number of pouches varies from 5 to 15. In Myxine the gill pouches do not open to the exterior like lamprays. In Myxine there are 6 pairs of pouches and in Eptatretus 13-15 pairs of pouches.
b. Septal Gills:
This type of gills are found in elasmobranchs (Fig. 6.78A). ‘Septal gills’ are called because the gill pouches are separated by the thick inter-branchial septa or partitions which are strengthened by tough fibrous tissue.
The gill chambers are longer than the pouch gills and communicate internally with the pharynx by larger spaces and also open to the outside by a comparatively narrow slit, the external gill cleft. The inter-brachial septa are very large that extend beyond the gill filaments.
c. Opercular Gills:
This type of gills occurs in bony fishes (Fig. 6.78B). ‘Opercular gills’ are called because a movable bony gill cover, the operculum contains the gills in an opercular cavity or gill chamber. The gill bar or septum is usually shorter than the elasmobranchs and may be absent.
Primary Respiratory Organs:
The primary respiratory organs of the fishes are the gills.
Typical Structure of Gill:
Typically each gill is a comb-like structure having series of gill- filaments attached to the gill-arch. Each gill-arch bears a double rows of gill-filaments. The surfaces of each gill-filament are thrown into numerous small folds which increase the sum total surface area of the gills for gaseous exchange. The respiratory area of the gill in fishes depends on the size and number of the gill-filaments.
The development of respiratory area depends on the habit of the fishes. In fast moving forms the respiratory area is more, while in slow moving or sedentary forms the respiratory area of the gill is lesser. The principle of gill-respiration is basically similar in all the fishes but the structure, number and orientation of gills vary considerably.
Location of Gills:
The gill-slits are usually short, but in Cetorhinus (huge Basking shark) these are very large and extend from the upper to the lower surface of the body. The first slit like aperture is designated as spiracle which is situated between the mandibular and hyoid arches. The second is called the hyoid an cleft which lies between the hyoid arch and the first branchial arch.
The rest of the gill-slits are located between the posterior branchial arches. In most of the elasmobranches, the number of the gill-slits is five on each side excluding the spiracle. But in some sharks the gill- slits may exceed the normal number of five pairs.
Hexanchus bears six pairs and Heptranchias possesses seven pairs of gill-slits in addition to the spiracles. In most of the elasmobranchs, particularly in the sharks, the gill-pouches open directly to the exterior by independent external gill-slits.
In the sharks the partitions between the gill- slits are prolonged backward as the folds of skin to cover the external gill-slits. In the bony fishes, the internal pharyngeal gill-slits are present, but these do not open independently to the exterior. All these open into a common branchial chamber which is covered by a movable gill-cover or operculum.
Each branchial chamber opens to the exterior by a large slit or aperture. The opercula of two sides may overlap or fuse ventrally. In some actinopterygians the degree of fusion becomes so great that the opercular openings become greatly reduced to small bilateral slits or round openings as observed in the eels.
The operculum is supported by broad, flat bony plates which may or may not be supported by slender branchiostegal rays on the ventral side.
Amongst the elasmobranchs, the holocephalans are operculate. These forms represent a stage which stands midway between the rest of the cartilaginous fishes and the bony fishes. In these forms, the inter-branchial septa are shorter than that in true sharks and the gills are lodged in a common branchial chamber.
The outer side of this chamber is bordered by a flap of skin resembling the operculum of the bony fishes. Each branchial chamber opens to the exterior by a single slit.
Function of Gills:
a. The gills in fishes are mostly respiratory in function.
b. Besides their respiratory role, the gills take part in the elimination of certain waste products and thus help in the maintenance of salt balance.
c. Both freshwater and marine teleosts excrete nitrogenous waste products through the gills in the form of ammonia and urea.
d. The gills of Cyprinus (Carp) and Carassius (Gold-fish) excrete the nitrogenous waste products six to ten times that of kidneys.
Types of Gills based on Location:
Depending on the location of the gills, two types of gills are found in fishes:
1. Internal Gills:
When the gills lie inside the gill-pouches (sharks) or in the branchial chamber (in holocephalans and bony fishes), these are called internal gills.
2. External Gills:
The larval stages of many fishes develop external gills for respiration. The external gills are of two varieties depending on their development.
(i) True External Gills:
These are independent of the internal gills and develop as the modification of the integument.
(ii) Prolongations from the Internal Gills:
These gills are located outside the body, but these are nothing but the prolongations of the internal gill- filaments which lie on the outer side of the body. True external gills occur in the young stages of Polypterus and Lepidosiren.
In the larvae of Protopterus and Lepidosiren, four pairs of small external gills are present. With the attainment of adulthood, the external gills in these forms are lost except Protopterus where vestiges of the external gills persist even in adult. In young Polypterus (Bichir) a pair of leaf-life external gills are present just above the gill-opening.
The external gills of the second category are found in the embryo of some selachians and some oviparous bony fishes. In the selachian embryos long filamentous external gills develop from the walls of the gill-slits and project out through the external gill-slits. These filamentous structures help in respiration because the sea-water circulates through these structures.
In the viviparous selachians, these structures also help to absorb nutrient. The external gills observed in the young’s of some egg laying bony fishes (i.e., Gymnarchus, Clupisdis and many others) are respiratory in function. In Gymnarchus, slender filaments from the internal gills project beyond the opercular edge and function as external gills.
Based on structure:
Based on structure and function, four types of gills are encountered in fishes. These are: hemi branch, pseudo branch, holobranch and lophobranch. The gill-pouches are separated by interbranchial septa. The anterior and posterior walls of each septum bear series of gill- filaments.
Each series of gill-filaments on one side of the interbranchial septum constitute the hemi branch or half-gill. The pseudo- branch is a modified form of the hemi branch.
The term pseudo branch is referred to the hemi branch which has lost the original respiratory function. A holobranch or complete gill is composed of two hemibranchs, i.e., a holobranch consists of an interbranchial septum (may be reduced in advanced fishes) with series of vascular gill-filaments developing on its anterior and posterior walls.
Another peculiar type of gill is observed in Seahorses and Pipe-fishes where the gill-filaments become greatly reduced to form rosette-like tufts. These tufts are small and are attached to the greatly reduced gill-arches. Such types of gills are called tuft gills or Lophobranchs.
Fate of Interbranchial Septa:
The interbranchial septa in the primitive fishes are thick and contain tough fibrous tissues. These are supported by many cartilaginous movable segments which constitute the gill-arches. The gill-arches are situated at the inner side of the septa. Each arch appears like a half-hoop.
The lowest pieces of the gill-arches almost join with the counterparts of the opposite and thus support the inner side of the pharynx like series of girders. The inter-branchial septa show the tendency towards reduction in course of evolution (Fig. 6.79).
In selachians, the interbranchial septa are arranged in such a fashion that a series of independent gill-slits are produced. In these forms the interbranchial septa are larger than the rows of gill-filaments. The interbranchial septa project backward as folds to cover the gill-slits.
This condition represents the primitive condition amongst the fishes. But in the remaining fishes these septa becomes reduced in a varying degree. In chimaeras, the inter-branchial septa are slightly shorter than the gill-filaments and the gill filaments project a little beyond the outer edges.
In the primitive bony fishes represented by Sturgeons (Acipenser) the septa become shorter and extend up to the midway. This condition is also observed in Labeo rohita and Tenualosa ilisah, but in the other bony fishes the septa become progressively shorter as seen in Salmon, Rita rita, Channa striatus etc.
The condition of the interbranchial septa in Sturgeons exhibits a transitional stage between the chondrichthyan and teleostean conditions. The rows of gill-filaments or the two sides of the septum are independent of one another but in Labeo rohita, the adjacent rows of gill- filaments are fused at the tips and bases so that a narrow slit-like aperture is left between these rows of gill-filaments.
Fate of Spiracles:
The spiracle is the slit-like aperture between the mandibular and hyoidean arches. This structure becomes subsequently modified in different fishes. In the sharks, the spiracles are present. The anterior side of the spiracular cleft bears spiracular gill composing of a number of gill-filaments. But in most of the fishes the spiracular gill is represented by a retemirabile or network of blood vessels called pseudo branch.
The pseudobranchs, with all probabilities, are the organs for special sense. In skates and rays, the spiracles are highly developed and are provided with movable valves. The external gill-slits are located ventrally and during rest on sandy bottom, there is every chance of the entry of sand particles inside the gill-pouches along with the respiratory water current.
So to prevent the clogging up of gill-filaments by the introduction of foreign particles, these fishes inhale water through the spiracles and expel it by way of the gill-slits. The spiracular openings are closed in adult holocephalans, although these are present in the larval stage. There is no evidence of the existence of pseudo branch in the spiracular opening. In sharks, the spiracles retain a few gill- filaments in adult stages.
In bony fishes, the spiracles are mostly absent, although spiracular pouches may develop. Amia and Lepisosteus lack the spiracle and the spiracular pouch is greatly reduced. In Acipenser, the spiracle and the tube-like spiracular pouch are present.
Scaphirhynchus lacks a spiracle. Polypters possesses a wide spiracle and a cellular ridge separates the spiracular pouch and the hyobranchial groove. In crossopterygians, the spiracles are absent. In Latimeria, the spiracular pouch is very deep, but in the lung-fishes, the spiracular pouch is greatly reduced.
The gill-rakers are specially developed on the inner edge of gill-arches. The gill-rakers are modified dermal denticles and are arranged in double rows. The development of gill-rakers in fishes depends on the particular mode of feeding. In the fishes which devour minute organisms, the gill-rakers are highly developed.
During swimming there is every chance of entry of the small creatures through the internal gill-slits and thus the gill-filaments may either be damaged or clogged. These are avoided by the development of the gill-rakers. The gill-rakers form a sort of sieving apparatus which strains water that bathes the gill- filaments.
The structure of the gill-rakers, varies greatly in different fishes (Fig. 6.80). The hering-like fishes (Hilsa, Cadusia, Gonialosa, Notopterus) are plankton feeders and in them the gill-rakers are slender and extremely elongated. These rakers form a close-set strainer. In many other filter feeders, the primary gill- rakers bear secondary and tertiary branches and thus appear like a fine gauze.
In Esox (Pike) the gill-rakers are reduced to bony knobs which prevent the entry of larger particles. The structure and number of gill-rakers vary considerably even within the closely related forms. In adult Alosa alosa, the lower limb of the arch bears about eighty gill-rakers, whereas Alosa fallax possess only thirty.
The gill-rakers in Cetorhinus (Basking shark) and Rhincodon (Whale shark) measure about 10-12 cm in length. These become flattened to form a closely set structure. This structure recalls the baleen plate of whalebone of whales in structure and function. The gill- rakers are generally absent in other sharks. In crossopterygians the gill-rakers are rudimentary.
Gills in Different Fishes:
The gills in fishes are basically similar. In elasmobranchs, the gills are mostly hemibranchs, whereas in the teleosts the gills are mostly holobranchs. The vestigial mandibular gills (pseudo-branch) are present in some fishes. Most of the sharks possess on each side a mandibular pseudo branch, a hyoidean hemi-branch and four, five or six holobranchs.
In holocephalans, the mandibular pseudo- branch is lacking, but the hyoid arch has a posterior hemi branch. The first, second and third gill-arches bear holobranchs whereas the fourth bears a hemi branch. In sharks, each gill-arch contains one afferent and two efferent vessels, while in holocephalans only one efferent vessel is present.
Amongst the Osteichthyes, the actinopterygians possess on each side, a mandibular hemi branch or pseudo branch, four holobranchs (on the first, second, third, and fourth gill-arches). In some forms, a hemi branch may be present.
A hyoidean hemi branch may be lacking in some cases. In crossopterygians, the nature of the gills is slightly different. There is no existence of mandibular pseudo branch in Latimeria, although small hyoidean hemibranchs may be present.
Like that of other actinopterygians, there are four pairs of holobranchs in Latimeria. But the fifth gill-arch is devoid of any gill. The dipnoans resemble Latimeria in gill arrangement. Neoceratodus contains a hyoidean hemi branch, holobranch on the fourth and fifth arches.
The sixth gill-arch bears an anterior hemi branch. In Lepidosiren, the second, third and fourth gill-arches have holobranchs. The hemibranchs are lacking.
It is generally considered that the pseudo branch in elasmobranchs and teleosts represents a modified posterior hemi branch of the mandibular gill-arch. They lack respiratory function.
The pseudobranchs become greatly reduced in different fishes. In Trout, the pseudo branch retails the characteristic comb-like appearance.
The gill-filaments become extremely reduced and are covered by the pharyngeal epithelium in case of Perch. In the Cod, the pseudo branch becomes completely covered in the pharyngeal epithelium to form a gland like organ called vaso-ganglion or rete mirabile.
Although the pseudobranchs are embedded deeply in the pharyngeal tissue, these structures retain the fine constituents of the gill-tissue. In Amia, the pseudobranchs are reduced and covered by pharyngeal mucous epithelium. In Catla catla, the pseudo branch (Fig. 6.81) is attached to the anterior gill.
The pseudobranchs may be free or may be covered by mucous epithelium. These receive oxygenated blood from the dorsal aorta. The pseudobranchs lack respiratory function in adults and sub-serve other functions. The mandibular hemibranchs usually help to close the spiracles and get oxygenated blood.
These structures may:
(i) Increase the oxygen concentration in the blood going to the brain or
(ii) Regulate the blood pressure in the ophthalmic artery or
(iii) Act as endocrine organ.
The pseudobranchs are made up largely of acidophilic cells. In Lepisosteus the mandibular pseudo branch remains in close contact with the hyoidean hemi branch and gets blood from the afferent hyoid artery and the efferent artery from the first arch.
Acipenser resembles Lepisosteus except that the mandibular pseudo branch and hyoidean hemi branch lack connection. The pseudobranchs of Amia lack direct afferent or efferent blood connection. The pseudobranchial vessel joins the orbital and ophthalmic vessels. Polypterus lacks mandibular pseudobranchs.
The hyoidean hemibranchs are present in most of the fishes. In the sharks, hyoidean hemibranchs are present. In selachians, the hyoidean hemibranchs have either disappeared or are represented by rudiments. In the holocephalans, hyoidean hemibranchs remain attached with the operculum.
But in most of the actinopterygians, the hyoidean hemibranchs are absent. In Acipenser, Lepisosteus and Polyodon these gills are present, but Amia lacks the hyoidean hemibranchs. In Scaphirhynchus, these hemibranchs are greatly reduced. Polypterus lacks both the mandibular pseudobranchs and hyoidean hemibranchs.
The absence of these gills may possibly be resulted due to the conversion of the swim-bladder into the air- breathing ‘lungs’. Amongst the crossopterygians, hyoidean hemibranchs are present in all the living forms except Lepidosiren. In Latimeria the hyoidean hemibranchs are small.
Blood Supply to Gills:
The gills are well-supplied with blood vessels. The flow of blood and the water current pass one another in opposite directions. This arrangement ensures efficient exchange of dissolved substances between the two fluids. If the direction of flow of the two fluids is experimentally reversed, the uptake of oxygen falls from 50% to 9%.
The gill-filaments are provided with folds which permit the blood to come in intimate contact with the water for gaseous exchange. The gills are composed of primary gill- filaments which produce numerous secondary folds (filaments). Each secondary gill-filament has a central core of vascular tissue over lined by a thin layer of connective tissue and mucous epithelium.
The vascular central core contains capillary networks and supporting Pilaster cells. In most of the teleosts, each gill- arch contains one afferent and one efferent branchial vessel. But in Labeo rohita, Clarias batraahus, Trichogaster fasciatus, Anabas testudineus and many others, two efferent vessels are present in each gill-arch.
Amongst the cartilaginous fishes, each gill-arch contains one afferent and bilateral efferent vessels. But in the holocephalans only one efferent vessel is present. In lung-fishes each gill-arch contains two efferent arteries.
In a typical teleost, each gill-arch contains an afferent branchial and an efferent branchial vessel. Each afferent branchial gives off primary afferent vessels to the primary gill-filaments. Each primary afferent vessel divides into a number of secondary and tertiary branches for the secondary gill-filaments. The afferent vessels break up into capillaries in the secondary gill- filaments.
These capillaries unite and the blood is carried to the primary efferent vessel. The primary efferent vessels run along the margin of the primary gill-filaments and get secondary efferent vessels from the secondary efferent vessels from the secondary gill-filaments.
Exchange of gases takes place while the blood passes through the capillaries and the oxygenated blood from the gill-filaments is collected in the main efferent branchial vessels.
Most of the fishes cannot survive out of water because of the failure of respiring in air. But there are many fishes which do survive out of water for a considerable period of time. This is possible by the development of certain specialised organs usually called accessory respiratory organs.
Type # 4. Circulatory System:
The structure of heart in fishes shows little variation (Fig. 6.93). In cartilaginous fishes (Fig. 6.93A), the heart consists of a sinus venosus, a ventricle, an auricle and a conus arteriosus. All the chambers of the heart are basically similar in fishes.
The size and shape of the component chambers may show variation. The conus arteriosus is gradually eliminated in course of evolution. Fig. 6.92 shows the structural variation of heart of some common teleosts.
The conus arteriosus proceeds forward as a stout tube, called ventral aorta. The number and arrangement of valves in the conus arteriosus are the most notable features in different fishes. In teleosts, the conus arteriosus is ill-developed and contains one or two sets of valves.
In some teleosts the conus is altogether unrepresented. In teleosts a large bulbus arteriosus is present at the base of the ventral aorta. The bulbus arteriosus is the dilated part of the base of the ventral aorta. It is not a part of the heart, but is a part of the arterial system. It forms the basal trunk of the main arteries.
In elasmobranchs and ganoid fishes (Fig. 6.93B) the valves in the conus arteriosus are numerous and are usually arranged in three longitudinal rows. In many elasmobranchs the auricle shows an advancement over other fishes, where it divides incompletely into left and right halves by an incomplete inter-auricular septum.
This modification is due to acquisition of aerial respiration. In many elasmobranchs there are six sets of valves. But reduction in the number of valves in the conus arteriosus is the common tendency in the evolution of fishes (Fig. 6.93). In dipnoans, the swim-bladder is transformed into the ‘lung’ and the oxygenated blood from the ‘lung’ is carried to the left auricle.
The general plan of the arteries and veins are more or less similar. The ventral aorta gives off afferent branchial arteries to the gills for oxygenation. The number of afferent branchial arteries corresponds directly to the number of gills.
After oxygenation in gills, the blood is returned to the dorsal aorta by efferent branchial arteries. The number and disposition of efferent branchial arteries show variation in different fishes. Fig. 6.94 shows in disposition of different main arteries in some teleosts.
The blood from the different parts of the body returns to the heart by paired anterior cardinal veins and unpaired or paired posterior cardinal veins and unpaired or paired posterior cardinals.
The venous system is basically similar in all fishes but minor variations are very common. The hepatic portal system collects blood from the alimentary canal, spleen, swim-bladder (when present) and gonads and finally emtifies into the liver. From the liver, the blood is carried to the sinus venosus by hepatic veins.
The fishes possess red blood corpuscles which contain haemoglobin. Some antartic marine teleosts belonging to the family Chaenichthyidae have colourless blood, i.e., the haemoglobin is lacking. The leptocephalus larval stage of the eel possesses colourless blood.
Type # 5. Nervous System:
The organisation of nervous system is basically similar in different fishes. The brain and spinal cord are covered by a single protective covering, called meninx primitiva. The spinal cord bears the dorsal fissure in most fishes, but the ventral fissure is lacking. The forebrain exhibits extensive variation. In most elasmobranchs, the cerebral hemispheres are widely separated, but in others the separation is not so well-marked.
In the latter forms, at least a median depression is present which indicates the division of the anterior part of the forebrain. In most teleosts, the prosencephalon is not divided into hemispheres and the roof is non-nervous and thin. The floor of the cerebral hemispheres (corpus striatum) is well-developed. In dipnoans, the cerebral hemispheres are quite distinct anteriorly.
The olfactory lobes are usually well-formed and are, in most cases, borne on long olfactory peduncles. In the rays and Acipenser, the olfactory lobes are solid structures. In general, the progression of telencephalon is limited not beyond the development of the olfactory centre. The diencephalon is moderately developed in fishes.
Presence of saccus vasculosus and inferior lobes, especially in elasmobranchs, are the modifications of the lower part of the diencephalon. The saccus vasculosus has folded and pigmented walls which act as a pressure receptor. It is well-developed in deep-sea fishes. The optic nerves in elasmobranchs form chiasma, but in teleosts the chiasma is absent where the optic nerves simply cross or decussates.
The cerebellum is poorly developed in sluggish fishes, but in active forms the cerebellum is prominent. Amongst the elasmobranchs, the cerebellum is elongated in sharks, but is comparatively shorter in rays.
The medulla oblongata exhibits structural modifications in response to different functional responses. In certain fishes, two well- developed vagal lobes are present in the medulla oblongata. These lobes are highly developed in many teleosts and are very prominent in buffalo fish (Carpiodes tumidus).
The electric lobes in electric rays are the spherical projections of the floor of the fourth ventricle and they innervate the electric organs. The origin and distribution of the cranial nerves are more or less similar in all the fishes.
Type # 6. Excretory System:
Adult fishes possess opisthonephros type of kidney. Although built on the same fundamental plan, the kidneys in fishes show great variation in shape. In most fishes, kidneys are of extremely elongated structures and extend the entire length of the body cavity.
They may attain voluminous size and the two kidneys show various degrees of fusion along the length. Usually the posterior sides of the kidneys are fused and the anterior ends remain free. The anterior part of kidney in almost all forms is non-renal.
In elasmobranchs, the kidneys in two sexes differ structurally. In males, the anterior non-renal part of the kidney is well-formed and sub serves reproductive function, but in females the anterior part of the kidney is functionless and degenerated.
In teleosts, the anterior portion of the kidney is converted into lymphatic tissue and does not perform any renal function. In teleosts there is no connection between testis and the kidney. Marine teleosts possess a fewer number of glomeruli than the freshwater forms. In some cases, the kidneys are wholly aglomerular. In toad-fish, Opsanus tau the kidneys are aglomerular.
Type # 7. Reproductive System:
Fishes practice several methods of reproduction. They are generally, bisexual but sexual dimorphism is usually absent excepting some fishes. The male sharks possess claspers. The brood pouch in Seahorse or Hippocampus and pipe-fishes (Syngnathus) are some of the striking features. Some fishes are hermaphroditic and parthenogenetic development occurs in some forms.
The sperms and ova are developed in separate gonads in majority of the fishes but the members of the families—Sparidae and Serranidae are hermaphroditic in true sense because the sperms as well as the ova are produced in the same gonad. In hermaphroditic fishes self- fertilization is a common occurrence. The gonads develop from the coelomic epithelium.
Male Reproductive System:
The testes are paired structures and are symmetrically placed in the anterior part of the coelom. The testes are attached with the body wall by mesorchia. The testes are roughly oval or elongated in shape. The testes often exhibit lobulated texture.
In dipnoans, the testes are enveloped by lymphoid tissue. The manner of transportation of the spermatozoa from the testis differs in many fishes. In most elasmobranchs, small efferent ductules arising from testis are connected with the kidney tubules of the anterior portion. In elasmobranchs, the connection of the efferent ductules from the testis and the archinephric duct occur in the anterior region of the opisthonephros.
This condition may be quite different in other fishes. In Polypterus, the kidney tubules at the posterior part of the kidney usually degenerate and sub serve reproductive function. In Protopterus and many teleosts, separations of the testis duct from the archinephric duct represent specialised condition.
In elasmobranchs, holocephalans and a few teleosts internal fertilization occurs and it is effected by copulatory organ. Clasping organs in elasmobranchs serve the purpose. In holocephalans, clasping organs of similar nature are present.
In some teleosts, the anal fin in males prolongs posteriorly into an intromittent organ, called the gonopodium. The gonopodium is present in the ‘guppy’, a common aquarium fish. The intromittent organs found in teleosts are actually the specialisation of haemal spines of some caudal vertebrae.
Female Reproductive System:
In fishes the ovaries are paired structures. In some fishes unpaired ovary is present, which is actually formed by the fusion of two ovaries. The ovaries are attached to the body wall by the mesenteries. The ovaries are asymmetrically placed in some forms. In many elasmobranchs, the right ovary becomes greatly developed and the left one atrophies.
In majority of the teleosts saccular ovaries are present and become greatly enlarged during breeding season. Other teleosts may possess solid ovaries. The structures concerned for the transportation of eggs from coelom to the exterior vary greatly in fishes. The members of the family Salmonidae and some other fishes the eggs escape through the abdominal pores.
In addition to the abdominal pores, ducts for transportation of eggs exist in most forms. In elasmobranchs, shell gland is present in each oviduct. The shell gland is better developed in oviparous forms than that in ovoviviparous species. In ovoviviparous dog-fish, Squalus acanthias, temporary shell surrounds several eggs in the uterus and the eggs undergo early development within it.
The teleosts have saccular ovaries and possess short oviducts. The two oviducts may often unite posteriorly to form a single duct which opens by genital pore. Most of the teleosts are oviparous, but ovoviviparous forms are also common. The homology of the oviducts in different forms is not certain.
All the elasmobranchs are oviparous excepting Rhinobatus and Trygonorhina which are viviparcus. Viviparity accompanies by formation of placenta is also present in teleosts. Viviparity is quite widespread in the families of the order Microcyprini.
Amongst elasmobranchs, viviparity accompanied by true placenta formation is quite common. In some sharks, the vascular chorion unites with the uterine wall. For the purpose of drawing nourishment from the maternal tissue, development of villi is also observed in Torpedo and Trygon.