FISHES. 273 



CLASS I. FISHES. 



FIG. 183. Perca fluviatilis. 



Pishes are organised for an aquatic life, and respire by 

 gills or branchiae. Their heart receives only venous blood 

 from the body, which it propels through the organs of res- 

 piration, to be submitted to the influence of the oxygen of 

 the water. The general circulation through their bodies is 

 maintained by a contractile dorsal vessel. 



The body of the fish is formed so as to give the least pos- 

 sible resistance to the medium it inhabits. It is furnished 

 with fins, which balance and guide its movements, the pro- 

 pulsion through the water being effected for the most part 

 by the action of the inflector muscles of the tail. The fins 

 consist of a fold of the integument, sustained by osseous or car- 

 tilaginous pieces, called rays, which are either articulated to 

 the vetebral column or to the bones of the anterior or poste- 

 rior extremities. Some of the fins are placed on the median 

 line of the back, abdomen, and tail, as the dorsal, anal, and 

 caudal. Others, as inspectoral and abdominal, are disposed 

 in pairs, and represent the extremities (fig. 183). The 

 bones of fishes are either osseous or cartilaginous, as they 

 have a greater or lesser proportion of earthy matter in their 

 composition. They have no medullary cavity. They contain 

 less gelatine, have a larger proportion of water, and are less 

 dense and compact in their texture than those of the higher 

 classes. The bones of fishes resemble those of the embryo 



