CLASS PISCES 445 



SUBCLASS II. DIPNOI. THE LUNG-FISHES. 



Family CERATODONTID^:. The Australian Lung-fishes. 

 Family LEPIDOSIRENID^E. The South American and 

 African Lung-fishes. 



3. THE ANATOMY AND PHYSIOLOGY or FISHES IN GENERAL 



External Features. FORM OF THE BODY. The body of 

 the majority of fishes is spindle-shaped and laterally com- 

 pressed, as in the perch a form that offers slight resistance to 

 progress through the water (Fig. 369). Variations in form are 

 correlated with the habits of the fish. For example, the flat- 

 fishes, or flounders, have thin bodies and are adapted for life on 

 the sea bottom; they are laterally compressed and swim on one 

 side or the other; the eels have a long cylindrical body which 

 enables them to enter holes and crevices; and the globe- fishes 

 when disturbed inflate themselves with air, becoming almost 

 spherical, in which condition they float in the water. The shape 

 of the head differs considerably among the fishes; in the angler- 

 fish it is enormous ; in the garpike it is long and pointed; and 

 that of the paddle-fish extends forwards as a thin paddle-like 

 structure. Many fishes, like the sea-horse (Fig. 398) and some 

 deep-sea species, are so curiously shaped as to show little resem- 

 blance to our common fishes. 



FINS AND TAIL. Fins arise in the embryo as median and 

 lateral folds of the integument (Fig. 375, A) which are at first 

 continuous. Later, parts of the folds disappear and the isolated 

 dorsal, caudal, anal, ventral, and pectoral fins persist (Fig. 375, 

 B). There is a theory that the paired fins arise from gill-arches, 

 but this method of origin seems less probable than that just 

 described. 



The ventral fins of fishes vary considerably in position, prob- 

 ably because their skeletal parts are held only by muscles. In 

 the perch (Fig. 368) they are situated beneath the pectoral fins 

 and are said to be ventral; in the fresh- water dogfish (Fig. 384) 

 they are just in front of the anus and are called abdominal; and 



