504 



CIIORDATA. 



loops of the gill circulation, ^\'hen these are tle\elopeil, atjcrcnt hrancliial 

 arteries, gill capillaries, and fijcrcut arteries can lie recognized, the latter 

 uniting to form the dorsal aorta and also gi\"ing off the arteries (carotids), 

 wliich go to the head. 



The iiephridia are a pair of large reddish-brown organs hing outside 

 the liody cavity to the right and left of the verteliral column, usually ex- 

 tending from heart to anus. Their ducts empt)' Kehind the aitus or in the 

 dorsal wall of the intestine and arc often provided with enlargements 

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



Fig. 554. — Heail of cmlir>-o telcost (diagram from GcgciibaurV a, auricle; ahr, 

 ventral aorta witli arterial arches: <;i/, dorsal aorta; r, carotid; dc. Cu\ierian duct, formed 

 by union of iugular and posterior cardinal veins; /;, nostril; .1, gill clefts; «', sinus veno- 

 sus; ;', \'entricle. 



morphologically from the urinary bladder of the higher vertebrates. The 

 gonatls, suspendeil liy mesorchia or mesovaria, are large aitil project into 

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

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

 in other forms by the pori alidomiitales or bv special ducts. A\"ith 

 the exception of the Dipnoi and gaitoids which ha\-e ait unec|ual total 

 cleavage, the eggs of fishes ha\-e a discoidal segmentation. 



Sub Class I. Elasmobranehii [Plagiostoiiii, Clioudroptervgii). 



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

 varying in length from a fool and a half to sixl}- feet, li\ing largely 

 on other vertebrates, ami noted for their voracity. Sontetintes slender 

 and cylintlrical (sharks, fig. 555), sometimes llaltened dorsoventrally 

 (skates, hg. 550), they agree in that the head is prolonged into a snout, sup- 

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

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

 end, and is trans\erse, (riagiostomi— 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 



