322 SYSTEMATIC ZOOLOGY. 



passing through the gill-slits. When the fish is out these 

 delicate filaments mat together, reducing the surface for 

 breathing; and then, too, the gills soon become dry, and 

 then are less favorable for the exchange of carbonic 

 dioxide and oxygen. 



Among the peculiarities of the skull are the numbers of 

 branchial arches and the ease with which these, the oper- 

 cular structures, and bones of the face can be separated 

 from the cranium (p. 293). In the Selachii these, like 

 the rest of the skeleton, are composed of cartilage. In 

 the Teleosts this is largely replaced by bone. Another 

 peculiarity is that the lower jaw does not directly join 

 (articulate with) the skull, but certain parts intervene 

 between the two, forming what is known as a suspensory 

 apparatus (see p. 294). 



The group of Pisces is divided into four subclasses: 

 Elasmobranchii, Ganoidei, Teleostei, and Dipnoi. 



SUBCLASS I. ELASMOBRANCHII (Selachii, Sharks, and 



Skates) . 



These forms, of which the dogfish is an example, are, 

 with few exceptions, marine. They are sharply marked 

 off from all other fishes by several characters. The 

 skeleton is entirely of cartilage, no bones being developed; 

 the body is usually covered with placoid scales (p. 318); 

 the gill-slits (five to seven in number) open separately to 

 the exterior, except in the Holocephali, and no operculum 

 is developed; the heart has an arterial cone, and the intes- 

 tine is provided with a spiral valve. There is usually a 

 spiracle, and the air-bladder is always lacking. The 

 mouth and nostrils are usually on the ventral surface. 

 The Elasmobranchs are, on the whole, the most primitive 

 of the jawed vertebrates and hence they have been studied 



