8o MARINE AND FRESHWATER FISHES 



thigh, shank, and foot, as obtains in such higher animals ; 

 the wrist-like development of the pectoral fins in the 

 Angler-fishes or Pediculati may be cited among the 

 nearest modifications in this direction. In place of this 

 the limbs are composed, for the most part, of a series of 

 soft parallel bony or cartilaginous rays invested by a 

 continuous expansion of the integument, and thus form 

 efficient paddle-like locomotive organs or fins, the fore and 

 hind pairs of these appendages being known respectively 

 as the pectoral and ventral fins. In addition to these paired 

 fins all fishes invariably possess a greater or less number 

 of median unpaired fins, these are the dorsal or back fins, 

 the anal or vent fins, and the caudal or tail fin. All of 

 these unpaired fins are supported by cartilaginous or bony 

 fin rays, which are joined to the body through the medium 

 of special spinous processes ; this structural constitution of 

 the median fins is especially characteristic of fishes, and 

 obtains in no other animals. The heart in all fishes, except- 

 ing Amphioxus, consists of a single auricle and ventricle, 

 the blood is cold and red, its component corpuscles being 

 distinctly nucleated, and of an oval shape. All fishes 

 respire the oxygen dissolved in the water by the means ol 

 gills or branchiae. These are supported upon a greater or 

 less number of bony or cartilaginous structures, the visceral 

 arches, developed immediately behind the head, and which 

 are brought into direct relation with the surrounding water 

 in front by the opening of the mouth, and behind by the 

 gill cleft or clefts. The skin, naked in some fishes, is more 

 usually covered with overlapping scales, or may be pro- 

 tected by a series of closely set bony plates, or by variously 

 distributed tubercles or spines. All fishes are dioecious 

 (bisexual), the majority being oviparous, but some, in- 

 cluding notably certain representatives of the Shark tribe, 



