Introduction
The spiny dogfish shark, Squalus acanthias, belongs to the Chondrichthyes, which first appeared in the Silurian Period and are among the earliest to branch off from the rest of the gnathostomes (jawed vertebrates).
Living chondrichthyeans comprise the sharks and rays (Elas- mobranchii, meaning plate-gilled) and the chimaeras (Holocephali, meaning whole or entire head). Among the specialized features that unite these groups are unique perichondral and endochondral mineralization, distinctive placoid scales, an inner ear that opens externally through the endolymphatic duct, pelvic claspers in males (an adaptation for the internal fertilization practiced by modern chondrichthyeans), and a cartilaginous skeleton. As the earliest agnathan vertebrates were bony, the cartilaginous condition of the chon- drichthyean skeleton is apparently derived. In addition to these specialized features, they retain numerous ancestral characteristics, which is one reason why they are ideally suited as subjects for the study of a basic vertebrate. Chondrichthyeans lack the swim bladder or lung that evolved early among the bony fishes, and modern sharks all possess a large, oil-filled liver that dramatically reduces their specific gravity.Modern chondrichthyeans are a fairly large and diverse group due mainly to the success of the sharks or Neoselachii, which are arranged in two groups, although it is not clear that they are sister groups. Sharks have fusiform bodies with well-developed paired fins and a powerful heterocercal tail. Five to seven branchial slits and, usually, a spiracle are present. A row of serrated, triangular, and pointed teeth commonly line each of the upper and lower jaws and are followed by rows of teeth that can rapidly replace broken or worn teeth. Efficient swimmers with a well-armed mouth, sharks are formidable slashing predators. However, not all sharks fit this mold.
Indeed, the largest of them, the basking and whale sharks, feed by straining food from the water.The skates and rays are closely related to the sharks and are placed in the Batoidea. They are dorsoventrally flattened, largely due to greatly expanded pectoral fins. The branchial slits open ventrally (the group is thus also known as Hypotremata, meaning under slits) and the prominent spiracle dorsally. The tail and caudal fin are generally reduced and often whip-like. Locomotion is accomplished through wave-like flapping of the fins rather than lateral undulations of the trunk and tail. Some rays can generate electric shocks, produced by modified muscles, to repel an attack or capture prey. The flattened condition of the body is typical of bottomdwelling forms. The teeth are modified into flattened plates for crushing bottom-dwelling small vertebrates, molluscs, and crustaceans. Like the largest sharks, however, the largest rays, such as the manta ray, tend to strain food from the water.
Holocephali, the other main chondrichthyean group, includes the chimaeras. Holocephalans are so called because they possess an operculum that covers the gills (thus there is a single opening on either side of the head, as occurs in bony fishes) and gives the head an undivided appearance. The peculiar globular head with dental plates and long, thin tail have inspired their alternate designation as ratfish. Additional features of chimaeras are that the upper jaw is fused to the braincase, a spiracle is present only as a transitory structure during ontogeny, the anterior vertebrae form a synarcual that articulates with an anterior dorsal fin, and the male has, in addition to pelvic claspers, a cephalic clasper. They also lack scales except for the modified spine at the base of the dorsal fin.