FINS. 45 



cally dried and hardened during the hot season, forms occur 

 entirely devoid of, or with only rudimentary, ventral fins 

 (Cyprinodon, Ophiocephalidse, Galaxiidse, Siluridae). The chief 

 function of these fins being to balance the body of the fish 

 wliilst swimming, it is evident that in fishes moving during a 

 great part of their life over swampy ground, or through more 

 or less consistent mud, this function of the ventral fins ceases, 

 and that nature can readily dispense with these organs 

 altogether. 



In certain fishes the shape and function of the fins are 

 considerably modified : thus, in the Eays, locomotion is almost 

 entirely effected and regulated by the broad and expanded 

 pectoral fins acting with an undulatory motion of their mar- 

 gins, similar to the undulations of the long vertical fins of the 

 Flat-fishes ; in many Blennies the ventral fins are adapted 

 for walking on the sea-bottom ; in some Gobioids {Perioph- 

 thalmus), Trigloids, Scorpsenioids, and Pediculati, the pectoral 

 fins are perfect organs of walking; in the 

 Gobies, Cyclopteri, and Discoboli the ventral 

 fins are transformed into an adhesive disk, 

 and finally in the Flying-fish, in which the 

 pectorals act as a parachute. In the Eels 

 and other snake-Kke fishes, the swimming 

 as well as the gliding motions are effected Fig. 9.— Ventrais 

 by several curvatures of the body, alternate °^ Gohms. 



towards the right and left, resembling the locomotion of Snakes. 

 In the Syngnathi (Pipe-fishes) and Hippocampi, whose body 

 admits of but a slight degree of lateral curvature, and whose 

 caudal fin is generally small, if present at all, locomotion is 

 very limited, and almost wholly dependent on the action of 

 the dorsal fin, which consists of a rapid undulating movement. 



The skin of fishes is either covered with scales, or naked. Skin and 

 or provided with more or less numerous scutes of various 

 forms and sizes. Some parts, like the head and fins, are more 



