ir. VERTEBRATA: PISCES. 



569 



hy mesorcliia or mesovaria, are large and i^roject into the body 

 cavity. Tliey are rarely unj)aired. In the elasmobranchs and 

 most ganoids these products pass out by the urogenital system (p. 

 552), in other forms by the jsori abdominales or by spiecial ducts. 



Cuvier divided tlie fishes into cartilagiuous and bony groups, an im- 

 portant step so far as the extremes (elasmobranchs and teleosts) were 

 concerned. Agassiz recognized a middle group which he named Ganoidei, 

 from the character of the scales, but his account was modified and made 

 more accurate by Johannes Muller, who also included the Dipnoi among 

 the fishes. At present the group of ganoids is retained largely as a matter 

 of convenience. Its members are more closely related with the teleosts 

 than with the elasmobranolis, and in America Ganoids and Teleosts are 

 united under the head Teleostomi, the name alluding to the presence of 

 a true upper jaw comparable to that found in higher vertebrates. 



Suh Class 1. Elasmohrancliii {Plafjiosfoini, Chondro'pteriiijii). 



The elasmobranchs, the shark-like fishes, are almost exclu- 

 sively marine, varying in length from a foot and a half to sixty 

 feet, living almost exclusively on other vertebrates, and noted for 

 their voracity. Sometimes slender aii.d cylindrical, as in the sharks 

 (fig. 50 ,s), sometimes flattened dorsoventrally, as in the skates (fig. 



Fig. ^ii^.—Acfinthtns viilf/aris,* doltish. (From Glaus.)"/?, vp.itral fin; /?r, pectoral flD ; 

 A-'.s', gill clefis ; n, nostril ; R\ R'-, dorsal fins ; i', heterocercal caudal fin ; !^iA. spiracle. 



Sliii), they agree in form in that the head is prolonged into a snout, 

 which is usually supported by a cartilaginous prolongation of the 

 cranium, the rostrum (fig. 588, A'). The mouth lies ventrally, at 

 more or less distance from the anterior end, and is transverse, 

 whence the naine Plagiostomi — transverse mouth. This position 

 makes it necessary that a shark appi'oaching its in-ej from below 

 must turn on its back before biting. The tail is heterocercal or is 

 drawn out in a long filament. The skin is covered with placoid 

 scales, usually close together, these being so small in some cases 

 that the skin — shagreen — is used instead of sandpaper for polish- 

 ing. More rarely the scales are larger, and the spines, which 

 project from the skin, justify in size and form the term dermal 

 teeth. Such strong spines occur especially at the front of the 



