CHAPTER XX 

 CLASS PISCES 



377. The class of fishes, representatives of which are famil- 

 iar to all, is important not only from the point of view of its 

 specialized present-day representatives but from the fact that 

 it was the first successful vertebrate group of geological times. 

 It represents the primitive aquatic habit from which the land- 

 inhabiting types of vertebrates must have arisen, and in it 

 we find the fundamental plan of structure which has been 

 modified in the higher forms (as the Amphibia) in adaptation 

 to their present mode of life. It must of course be borne in 

 mind that the types of fishes which are supposed to be the 

 ancestors of the air-breathing vertebrates were much less 

 specialized in structure than the present members of the class. 

 There are, however, even now some of the fishes which have 

 changed less than the majority, from the primitive condition. 



378. General Characteristics. — 



1. Aquatic vertebrates having gills functional throughout 

 life. Gills consisting of filaments or sheets, containing blood- 

 vessels and attached to bony or cartilaginous arches in the region 

 of the pharynx. 



2. Paired appendages (pectoral and pelvic) normally fin- 

 like, — not having a median jointed axis as in the limbs of the 

 higher vertebrates. Medial fins dorsal, ventral, and caudal, 

 the latter being the chief organ of locomotion. 



3. There is usually a dermal skeleton consisting of scales, 

 covered with epidermis. The latter may deposit enamel on 

 the dermal core of the scale. 



4. Heart two-chambered containing systemic (impure) blood. 



5. Vertebral column either cartilaginous or bony; the verte- 

 brae biconcave. 



359 



