504 



CHORD ATA. 



loops of the gill circulation. When these are developed, afferent branchial 

 arteries, gill capillaries, and efferent arteries can be recognized, the latter 

 uniting to form the dorsal aorta and also giving off the arteries (carotids), 

 which go to the head. 



The ncphridia are a pair of large reddish-brown organs lying outside 

 the body cavity to the right and left of the vertebral column, usually ex- 

 tending from heart to anus. Their ducts empty behind the anus or in the 

 dorsal wall of the intestine and are often provided with enlargements 

 called, from their functions, urinary bladders, although totally different 



FIG. 554. TTcad of embryo teleost (diagram from Gegenbaur). a, auricle; abr, 

 ventral aorta \vith arterial arches; ad, dorsal aorta; c, carotid; dc, Cuvicrian duct, formed 

 by union of jugular and posterior cardinal veins; n, nostril; s, gill clefts; sv, sinus veno- 

 sus; v, ventricle. 



morphologically from the urinary bladder of the higher vertebrates. The 

 gonads, suspended by mesorchia or mesovaria, are large and project into 

 the body cavity. They are rarely unpaired. In the elasmobranchs and 

 most ganoids their products pass out by the urogenital system (p. 487), 

 in other forms by the pori abdominales or by special ducts. With 

 the exception of the Dipnoi and ganoids which have an unequal total 

 cleavage, the eggs of fishes have a discoidal segmentation. 



Sub Class I. Elasmobrancliii (Plagiostomi, Cliondropterygii}. 



The elasmobranchs, the shark-like fishes, are almost wholly marine' 

 varying in length from a foot and a half to sixty feet, living largely 

 on other vertebrates, and noted for their voracity. Sometimes slender 

 and cylindrical (sharks, fig. 555), sometimes flattened dorsoventrally 

 (skates, fig. 556), they agree in that the head is prolonged into a snout, sup- 

 ported by a cartilaginous prolongation of the cranium, the rostrum (fig. 

 546, R). The mouth is ventral, at some distance from the anterior 

 end, and is transverse, (Plagiostomi transverse mouth). This makes 

 it necessary that a shark approaching its prey from below must turn on its 

 back before biting. The tail is heterocercal or is drawn out in a long 



