Introduction
The Chondrichthyes, also called elasmobranchii, comprise sharks, rays, skates, chimaeras, etc. Fishes are essentially aquatic and jaw-bearing true vertebrates. This class is specially known for their unimaginable number (about 40,000 species) and bewildering forms. They are characterized by having streamlined bodies, covered with scales. The presence of gills for respiration and paired fins for swimming. They belong to the superclass Pisces which can be sorted out into two distinct evolutionary lines cartilaginous Fishes of Chondrichthyes (Gr. chondros, cartilage + ichthys, fish) and bony fishes or Osteichthyes (Gr. osteon. bore).
General characters
Sharks (sub-order Pleurotremata) are usually somewhat fusiform and slightly compressed laterally. Rays occupy a separate sub-order (Hypotremata), but despite their radical dorso-ventral compression, the migration of the eyes to the dorsal surface, and their apparent dissimilarity from the sharks in other ways, the two groups have many features in common.
The flattened form of the rays is an adaptation to a bottom-dwelling existence. They can commonly be seen gliding along the bottom of all relatively warm seas, and they extend into circumpolar regions as well. Although they are popularly supposed to be confined to shallow waters, some inhabit great depths and are blind.
The elasmobranchii head is in many cases produced forwards into a long rostrum. This is of great length and bordered with triangular teeth in the sawsharks (Pristiophorus) and sawfishes (Pristis). In the Hammerhead Shark (Sphyrna = Zygaena) the anterior part of the head is extended transversely and the eyes are carried at the lateral extremities.
There are well-developed median and paired fins. The caudal fin is large and, as a rule, strongly heterocercal in the sharks and shark-like rays. It is reduced in most of the latter group. The dorsal and anal fins are large in the sharks, the former completely divided into two. In the rays the dorsal fin is usually small, and the anal fin absent. The paired fins differ widely in the two groups.
The mouth is situated on the ventral surface of the head, usually a considerable distance from the anterior extremity. In front of each angle of the mouth is the opening of one of the olfactory sacs.
The integument develops any hard parts as is the case in the majority of the Chondrichthyes, they take the form, not of regular scales, as in most other fishes, but of numerous rough, hard placoid scales. These vary greatly in shape, are usually extremely minute, but are in some cases developed, in certain parts of the surface, into prominent tubercles or spines.
The endoskeleton is composed of cartilage, often with a deposition of calcareous matter in special places- notably in the jaws and the vertebral column.
The skull is an undivided mass of cartilage, hardened, in many cases, by deposition of calcareous matter, but not containing any true bony tissue. It consists of a cartilaginous case which protects the brain and the organs of special sense.
The arrangement of the muscles is simple. The trunk-muscles are divided into a pair of dorsal and a pair of ventral divisions each, composed of many myomeres with intercalated myocommata, following a metameric arrangement. The ventral part, where it forms the muscles of the wall of the abdominal cavity, is composed externally of obliquely running fibres, and represents one of the two oblique muscles of the abdomen of higher forms.
Electric organs occur in several elasmobranchs. They are best developed in the electric-rays, numb-fishes, or torpedoes, in which they form a pair of large masses running through the entire thickness of the body, between the head and the margin of the pectoral fin.
Alimentary Canal and Associated Structures -Teeth are developed on the palatoquadrate and on Meckel’s cartilage. They are arranged in several parallel rows, and are developed from a groove within the margin of the jaw. Successive rows come to the front, and, as they wear out, fall off and are replaced by others. In the sharks the teeth are usually large, and may be long, narrow, and pointed, or triangular with serrated edges, or made up of several sharp cusps.
In the rays the teeth are more or less obtuse. Sometimes, as in the eagle-rays, they form a continuous pavement of smooth plates covered with enamel. Thus they are adapted to crushing food such as molluscs, crabs, and other animals.
The respiratory and associated organs of the Chondrichthyes always have the general structure and arrangement already described in the case of the The respiratory and associated organs of the Chondrichthyes always have the general structure and arrangement already described in the case of the Dogfish. In the rays the water for respiration is taken in mainly through the spiracles, in the sharks through the mouth.
Blood-vascular System – The heart has, in all essential respects, the same structure throughout the group. The conus arteriosus is always contractile, and contains several rows of valves. The general course of the circulation is the same in all, with some variation in the precise arrangement of the vessels. In some of the rays the ventral aorta and the roots of the afferent vessels are partly enclosed in the cartilage of the basi-branchial plate.
Brain -The fore-brain greatly exceeds the other divisions in size. In Scymnorhinus there are two widely-separated cerebral hemispheres containing large lateral ventricles. In other genera there is at most, as in dogfishes, a median depression of greater or less depth, indicating a division of the anterior end of the fore-brain into two lateral portions.
Urinogenital Organs and Reproduction -The kidneys differ somewhat in the two sexes. In the male the anterior portion persists in the epididymis, and its duct becomes the spermiduct, while the posterior portion, which is the functional kidney, has a duct or ducts of its own. In the female selachian there is no direct connection between the reproductive and renal organs.
the anterior portion of the kidney may be functional, and its duct persists, opening along with those of the posterior portion. In the male the urinary ducts open into a median chamber-the urinogenital sinus-which extends into the cloaca, and receives also the spermiducts. It communicates with the general cavity of the cloaca by a median opening situated on a papilla-the urinogenital papilla. In the female there is a median urinary sinus, into which the urinary ducts open, or the latter may open separately into the cloaca.
Development – Except in one species of Heterodontus, cleavage is meroblastic. It is confined to the germinal disc. A segmentation cavity appears early between the blastoderm cells and the yolk; the floor of the blastocoele becomes covered with a syncytium which probably plays a part in yolk digestion. When cleavage is complete the blastoderm is a lens-shaped disc thicker at one end-the future caudal extremity.
Mass migrations of cells now take place round the dorsal lip, which is at the posterior edge of the blastodisc. An archenteron is formed . (An archenteron is not formed in the teleosts, but in most other respects gastrulation is similar in both groups) The invaginated cells migrate forward forming mesodermal and endodermal structures and eliminating the blastocoele at the same time. It is possible that in addition to this forward migration some endodermal cells are derived in selachians by delamination from the early gastrula.