PISCES. 



37 



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. 15). In the Selachii these, like the 

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

 Teleosts this is largely replaced by bone. Another pecu- 

 liarity is that the lower jaw does not directly join (articu- 

 late with) the skull, but certain parts intervene between 

 the two, forming what is known as a suspensory apparatus. 



The group of Pisces is divided into five subclasses. 



SUBCLASS I. SELACHII (p. 21). 



SUBCLASS IL HOLOCEPHALT. 



A group of less than ten species of strange marine car- 

 tilaginous fishes in which the upper jaw is firmly united to 

 the cranium, the gills are covered by an operculum, and a 

 spiracle is lacking. Mouth and nostrils are ventral, as in 



FIG. 15 Chimcera monstrosa. 



the sharks. The name Chimaera, given to some forms, em- 

 phasizes their strange appearance, 



