422 Comparative Morphology of Chordates 



various groups which flourished in Paleozoic times and became highly 

 specialized in their several peculiar ways, but they all died out without 

 giving rise to any race of fishes capable of going 011 into the succeed- 

 ing ages. 



A comparatively simple classification of modern fishes may be 

 arranged as follows: 



Class Pisces 



A. CHONDRICHTHYES 



subclass i. elasmobranchii (selachii) : sharks and rays (skates) 

 subclass ii. holocephali: Chimaera 



B. OSTEICHTHYES 



SUBCLASS III. CROSSOPTEBYGII : "lobe-fills" 



subclass rv. actinoptebygii : "ray-fins" 



Order Chondrostei : sturgeons 



Order Polypterini : Polypterus 



Order Holostei: gar pike (Lepidosleus), Amia 



Order Teleostei: common "bony fishes" 

 subclass v. dipnoi : lungfishes 



SUBCLASS I. ELASMOBRANCHII 



Sharklike Fishes. Endoskeleton cartilaginous (Fig. 325). 

 Well-developed vertebral column, with only vestiges of notochord 

 between successive biconcave (amphicoelous) vertebrae (Fig. 326). 

 Caudal fm heterocercal — i.e., two-lobed — the vertebral column bend- 

 ing upward into the larger dorsal lobe (Fig. 327C). Upper jaw usually 

 suspended movably below the cranium (Fig. 116). Skin usually 

 thickly beset with small placoid scales, each consisting of a basal 

 plate surmounted by one or more sharp projecting spines (Fig. 328). 

 Respiratory passages consist of spiracles (closed in a few sharks) and 

 five to seven pairs of branchial clefts which open independently and 

 directly to the exterior. Internal surface of intestine increased by 

 presence of an extensive spiral fold of the inner layer of the tube — the 

 "spiral valve" (Fig. 23). Urinogenital ducts and anus open into a 

 common cloaca which has a single external aperture. 



The elasmobranchs are marine, only rarely occurring in fresh 

 water. They inhabit the warmer and temperate waters of all seas. The 

 sharks (Fig. 329A) and dogfishes are powerful swimmers, of roving 

 habits, and predaceous. The rays or skates (Fig. 330) are anatomically 

 sharklike except in the extreme dorsoventral flattening of the body and 

 exaggeration of the pectoral fins — a form related to their sluggish 

 bottom-dwelling habit. 



