elasmobranch

…further grouped into one subclass, Elasmobranchii, and the chimaeras into another, Holocephali. Some authorities classify the elasmobranchs into one class (Selachii) and classify the chimaeras into another (Holocephali); however, assigning the two groups class rank implies a degree of distinctness equal to that of the amphibians (Amphibia), reptiles (Reptilia), birds…

Subclass Elasmobranchii (sharks and rays) Numerous teeth derived of placoid scales; 5 to 7 gill clefts; operculum absent; cloaca; upper jaw not fused with braincase; dorsal fin nonerectile; with spiracles; worldwide distribution. Subclass Holocephali (chimeras) Teeth fused to bony plates; no scales; 4 gill

Subclass Elasmobranchii Chondrichthians with 5–7 pairs of gill clefts not covered by a fold of skin, opening separately to the exterior. Order Selachii (sharks) Elasmobranchs with gill clefts opening at least partly on the side of the body. More than 400 species. Order

Class Chondrichthyes Subclass Elasmobranchii (sharks and rays) Chondrichthyans with 5–7 pairs of gill clefts not covered by a fold of skin, opening separately to the exterior. Order Selachii (sharks) Elasmobranchs with gill clefts opening at least partly on the side of the body.

All cartilaginous fishes—the elasmobranches (e.g., sharks, rays, and skates)—employ internal fertilization and usually lay large, heavy-shelled eggs or give birth to live young. The most characteristic features of the more primitive bony fishes is the assemblage of polyandrous (many males) breeding aggregations in open water and the absence…

In cyclostomes (lampreys and hagfish), elasmobranchs (sharks, skates, and rays), and teleosts most of it differentiates, and the gonads extend nearly the length of the body trunk. In tetrapods (amphibians, reptiles, birds, and mammals), the cranial portion, at the anterior end, generally does not differentiate; in toads only the more…

In viviparous elasmobranchs development takes place in the uterus, the lining of which develops parallel ridges or folds covered with villi or papillae (trophonemata) that constitute a simple placenta (site of fetal–maternal contact). In contact with this region is the yolk sac of the embryo, which serves…

…ancestors of the cyclostomes and elasmobranchs had armoured headcases, which served largely a protective function and appear to have been true bone, modern cyclostomes have only an endoskeleton, or inner skeleton, of noncalcified cartilage and elasmobranchs a skeleton of calcified cartilage. Although a rigid endoskeleton performs obvious body supportive functions…

In elasmobranch and ganoid fishes a pair of spiracles, derived from the gills, is used as a water passageway during respiration. The nasal opening of whales and other cetaceans is called a spiracle, as is the respiratory opening behind the eyes of rays and skates. In…