500 PISCES FISHES. 



(62) ; their office and efficiency as organs of mastication must be 

 obvious to the most superficial observer. 



Upon reviewing the general disposition of the skeleton in one of 

 the osseous fishes, it is at once apparent that the great instru- 

 ment of locomotion is the tail, which by extensive and vigorous 

 lateral movements sculls the body rapidly along through the 

 yielding element in which these creatures live. In the construc- 

 tion of the caudal extremity of the skeleton, every precaution has 

 evidently been taken to convert this part of the body into a broad 

 and expanded oar, possessed of the utmost possible flexibility in 

 the lateral direction. No pelvis, therefore, trammels the move- 

 ments of the spine, neither do any transverse processes limit the 

 extent of flexion from side to side ; while, on the contrary, the 

 extraordinary developement of the spinous processes both above 

 and below, and more especially the vertical caudal fin, give an 

 extent of surface proportioned to the wants of the animal. 



The dorsal and anal fins, situated upon the mesian plane, 

 steady, and perhaps in some measure direct, the movements of the 

 body ; while the arms and legs, or rather the pectoral and ventral 

 fins, which are in this case of secondary importance as locomotive 

 instruments, exhibit a very rudimentary condition, and are but 

 feeble agents in progression. 



The posterior extremities, or ventral fins, are even less efficient 

 than the pectoral in this respect ; and their position is found to vary 

 remarkably in different orders. In the Perch these organs are, 

 as we have seen, attached to the bony framework of the shoulders. 

 In the Carp tribe (Cyprinidse) they are removed far back towards 

 the commencement of the tail, and the bones supporting them are 

 merely embedded in the muscles of the abdomen. In the Cod 

 (Gadidse) the legs are absolutely in front of the arms, being sus- 

 pended under the throat ; and in the Anguilliform fishes, the Eel 

 for instance, the ventral extremities are altogether wanting. 



(538.) Such being the imperfect developement of the usual 

 locomotive organs, we are quite prepared to expect a corresponding 

 modification in the disposition and efficiency of different parts of 

 the muscular system. When we compare the muscles of a fish 

 with those of any of the higher Vertebrata, the contrast is indeed 

 very striking. 



Delicate muscles (Jig. 224) are provided for the erection or de- 

 pression of the different rays sustaining the dorsal and ventral fins, 

 and thus the fins themselves are expanded or folded up at pleasure. 



