CHAPTER XX. 

 CLASS I PISCES. 



370. The class of fishes, representatives of which are 

 familiar 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 condi- 

 ton. 



371. General Characteristics. 



1. Fishes are aquatic vertebrates having gills functional 

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

 taining 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. 



355 



