10 LOCOMOTION OF FISHES. 



this passage, for it rather seems to be the product of a secretion, 

 having its seat in a glandular portion of the walls of the reservoir- 

 itself; and sometimes this is completely closed. By the move- 

 ments of the ribs, this elastic vessel is more or less compressed ; 

 and, according to the space that it occupies, it gives to the body 

 of the fish a specific gravity, equal, superior, or inferior, to that 

 of the water ; and thus enables it to remain in equilibrium, to 

 descend, or to rise, in this liquid. It has been remarked that it 

 is often absent, and that it is generally very small, in the species 

 destined to swim at the bottom of the water, or even to bury them- 

 selves in the mud, such as the Rays, Soles, Turbots, and Eels. 



528. Amongst a small number of Fishes, the pectoral fins 

 have an extreme 



development, and 

 thus permit the 

 animal to support 

 itself for some 

 minutes in the 

 air after it has 

 leaped out of the 



FIG. 251. DACTYLOPTBRUS, ONE OF THE FLYING FISH. 



water. The Dac- 



tylopterus affords an example of this construction. There are 

 some, which, by crawling, or by repeated leaps, can advance 

 upon the ground. Some have been mentioned which can climb 

 trees ; but these examples are very rare. 



529. In treating of the organs for movement amongst Fishes, 

 we must not omit to mention a very singular apparatus, which 

 is seen in some of these animals, and which enables them to 



adhere with great firmness to foreign bodies. 

 This is a flattened disc, which covers the upper 

 part of the head, and which is composed of a 

 certain number of cartilaginous and moveable 

 plates, directed obliquely backwards (Fig. 252). 

 The Fish of the genus Echineis are the only 

 FIG. 252. SUCKING- species which present this mode of organisation ; 

 D,scoFTH E RBMORA. and one O f fo^ ^fo^ lives in the Mediter- 

 ranean and in the Atlantic, and which has been for a long time 



