436 VERTEBRATE LIFE AND ORGANIZATION 



eggs are laid and develop externally. Skates are oviparous, but there is 

 no free larval stage, as there is in Irogs and many other oviparous am 

 mals. The eggs are very heavily laden with yolk and the embryos develop 

 within the protective capsule. A lew sharks are also oviparous, but mosf 

 have clei>artc(l Ironi this primitive egg-laying habit. In the dogfish, tor 

 example, the lertilized eggs are retained in a modified portion of the 

 oviduct known as the uterus. Each embryo derives its food in part from 

 yolk within the egg and in part from the mother's blood stream by 

 means of a primitive yolk sac placenta in which blood vessels in the 

 wall of the embryo's )i>lk sac are in direct contact with vascularized 

 flaps of the uterine lining. Food, gases and possibly other materials 

 diffuse between the mother and embryo, but no blood exchange occurs 

 between them. This is an example of viviparous reproduction, for the 

 embryo derives a large jjart of its nutrients from the mother's blood 

 stream, and the young fish is born in an advanced stage of develop- 

 ment as a miniature adult. Still other sharks are ovoviviparous; the egg 

 is retained within the mother's reproductive tract but most of the em- 

 bryo's nutrients come from yolk stored within the egg. It is frequently 

 difficult to make a sharp distinction between viviparous and ovovivi- 

 parous reproduction, for the young are born at an advanced stage of 

 development in both cases. 



199. Evolution of Cartilaginous Fishes 



The ancestral Chondrichthyes were essentially shark-like, but 

 in their subsequent evolution the cartilaginous fishes have diverged 

 widely, and have become adapted to many modes of life within the 

 aquatic environment. One line of evolution (subclass Holocephali) has 

 led to our present-day, rather rare, deep-water ratfish (Chimaera) (Fig. 

 22.7 D). In these fishes, the gill slits are covered by an operculum so 

 there is a common external orifice, and the tail is long and ratlike. The 

 other line of evolution (subclass Elasmobranchii) is distinguished by 

 having separate external openings for each gill slit. Elasmobranchs have 

 been far more successful, and have diverged into two contemporary 

 orders— Selachii (sharks and dogfish) and Batoidea (skates and rays). 



Sharks. Most selachians are active fishes that feed voraciously with 

 their sharp, triangular-shaped teeth upon other fishes, crustaceans and 

 certain molluscs. Although there are many records of sharks attacking 

 and killing man in the warmer seas, most species will not do so, and 

 there is little danger to swimmers in temperate waters. The largest 

 sharks, such as the whale shark {RJiincodon) which may reach a length 

 of about 50 feet, have minute teeth, and feed entirely upon small crus- 

 taceans and other organisms that form the drifting plankton of the 

 surface layers of the ocean. They gulp mouthfuls of water, and as the 

 water passes out of the gill slits, the food is kept in their pharynx by 

 a branchial sieve. Whale sharks are the largest living fishes. 



Skates and Rays. Skates and rays are bottom-dwelling fishes that 

 are flattened dorsoventrally, and have enormous pectoral fins whose 

 undulations propel the fish along the bottom (Fig. 22.7 C). Their 



