STRUCTURE OF THE VERTEBRATES 37 



includes the largest known fish, some reaching a length of sixty 

 feet, even these animals lack bone in the skeleton. There are, 

 however, bony scales as outgrowths of the skin. These are the 

 forerunners of the dermal or skin plates of the higher groups. 

 The teeth, which are found first in the elasmobranchs, are con- 

 tinuous with the scales, and are modified dermal denticles. The 

 mouth is ventral in position. 



The sub-class is divided into several extinct orders, and two 

 recent orders. Of the former, only the type genus of one order 

 will be discussed in this text. This is Cladoselache, an ani- 

 mal that gives an indication as to the evolution of the fins. One 

 of the living orders (Holocephali) may be considered an end 

 group and far from the line of evolution of the higher types. 

 The remaining order, the Selachii, are divided into two sub- 

 orders, one including the sharks, the other the skates and rays. 



The elasmobranchs, like the higher fish and the reptiles, have 

 two methods of reproduction. The eggs are large yolked and fer- 

 tilized internally. However, some of the selachians secrete a 

 horny shell around the egg and discharge them into the water. 

 These elasmobranchs are oviparous, or egg laying. Other mem- 

 bers of the order retain the eggs in the oviducts until the embryo 

 is capable of independent existence, at which time the young 

 fish is discharged into the water. This condition is called ovo- 

 viviparous to distinguish it from the viviparous development of 

 the mammals. In ovo-viviparous animals the young depends 

 upon the yolk for its nourishment, and grows by drawing on 

 the food stored in the egg. The mammalian development will be 

 discussed under the heading Mammalia. 



The skates and rays are a specialized group, and are of inter- 

 est to the naturalist as showing the adaptive radiations of the 

 order. An almost complete series can be made showing the 

 course of evolution from the sharks. Beginning with a shark- 

 like skate, these animals have become flattened dorso-ventrally 

 and proportionately widened. In this process the pectoral fins 

 have become fused to the body w^all, and the tail has degen- 

 erated until it is only a whip-like structure. Specimens of the 

 ray have been captured measuring more than twenty-five feet 

 across. These fish are an interesting offshoot of a primitive 

 stock. 



