IV. VERTEBRATA : PISCES. 569 



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. 

 552), in other forms by the pori abdominales or by special ducts. 



Cuvier divided the fishes into cartilaginous 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 Miiller, 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 elasmobranchs, 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. 



Sub Class I. Elasmobranchii (Plagiostomi, Chondropterygii). 



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 and cylindrical, as in the sharks 

 (fig. 598), sometimes flattened dorsoventrally, as in the skates (fig. 



Spl 



FIG. 598. Acanthias vulgaris* dogfish. (From Claus.)*"/?, ventral fin; Br, pectoral fin; 

 A'.s, gill clefts ; n, nostril ; R', R*, dorsal fins ; 6', heterocercal caudal fin ; Spl, spiracle. 



599), 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, R). The mouth lies ventrally, at 

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

 whence the name Plagiostomi transverse mouth. This position 

 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 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 



