DORSAL FINS. 23 



except, indeed, that the caudal fin represents the tail 

 feathers of the latter, but not the true tail of the 

 former, which is an actual continuation of the verte- 

 brae. The dorsal, after the pectoral, seems to be the 

 most essential for the aquatic economy of fishes, be- 

 cause there are only a very few instances yet known 

 where it is entirely wanting, and all these occur in that 

 order where the fins gradually disappear, and nature 

 passes into the marine worms. There seems to have 

 been a notion that the office of the dorsal was to pre- 

 serve the fish in a perpendicular position; but some 

 recent experiments does not sanction this idea, and 

 there is every reason to suppose that this object is 

 really effected by the pectorals, which, being placed 

 symmetrically, one on each side, preserve the body in 

 equilibrium. Besides, it is quite clear, that if this pur- 

 pose could only be effected by the dorsal, it would 

 follow that such fish as the Gymnotus hrachiurus and 

 its allies, where this fin is altogether wanting, could not 

 swim at all. Yet these are compressed fishes, and, 

 therefore, obviously intended for a perpendicular posi- 

 tion ; and they all have pectorals. Dorsal fins will 

 now be viewed as regards their construction, number, 

 form, and disposition. 



(26.) The construction of the dorsal is so far like 

 the other fins we have been describing, that it is gene- 

 rally composed of rays, connected, either partially or 

 entirely, by a membrane : but then the nature of these 

 rays varies in the different groups ; and in certain fa- 

 milies, where there are two dorsal fins, the hinder one 

 is adipose, that is, resembling a thick fleshy lobe, 

 attached to the back, and covered by the common skin, 

 in which neither rays nor membrane can be distinguished. 

 Fins of this description, with but one exception yet dis- 

 covered, are confined to the soft-rayed fishes. Native 

 examples occur in the salmon family : while among the 

 SiluridcB, or cat-fish, these fins are almost universal. 

 The Gadiadce, or cods, show us the next advance towards 

 a more organised construction : the fins, indeed, are 



c 4j 



