CHAPTER XX 



CLASS I. 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 Amphibians) 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. Fishes are aquatic vertebrates having gills functional 

 throughout life. These consist of filaments or sheets, contain- 

 ing blood-vessels and attached to bony or cartilaginous arches 

 in the region of the pharynx. 



2. Paired appendages (pectoral and pelvic) are normally 

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

 the higher vertebrates. The medial fins are dorsal, ventral, 

 and caudal. The last is 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. There is a two-chambered heart through which the sys- 

 temic (impure) blood flows. 



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

 brae biconcave. 



358 



