60 ADAPTATION OF THE FISH'S SKELETON. 



forwards close to the head. The absence of any neck 

 gives the advantage of a more extensive and resisting 

 attachment of the head to the trunk, and a greater pro- 

 portion of the trunk is left free for the allocation of the 

 muscular masses which move the tail. In the " caudal" 

 division of the vertebral column, the parapophyses cease 

 to extend outwards; they bend downwards, unite and 

 elongate in that direction, proportionally with the elon- 

 gation of the spines above, whilst dermal and intercalated 

 spines shoot forth from the middle line above and below, 

 giving the vertically extended, compressed form to the 

 hinder half of the body, by the alternating lateral strokes 

 of which the fish is propelled forwards in the diagonal 

 between the direction of those forces. The advantage of 

 the biconcave form of vertebra, with intervening elastic 

 capsules of gelatinous fluid, in producing a combination 

 of the resilient with the muscular power, is as obvious as 

 it is beautiful to contemplate. 



The fixation and coalescence of any of the vertebra in 

 this locomotive part of the fishes' body, analogous to the 

 part called "sacrum" and "pelvis" in land quadrupeds, 

 would be a great hindrance to the alternate and vigorous 

 inflections of that part, by which mainly the fish swims. 

 A " sacrum" is a consolidation of part of the vertebral 

 axis of the body, for the transference of more or less of 

 ;the weight of that body upon limbs organized for its sup- 

 port on dry land ; such a modification would have been 

 not merely useless, but a hinderance to a fish. The pec- 

 toral fins are the prototypes of the fore-limbs of the higher 

 vertebrates. With their terminal segment, or "hand," 

 alone projecting freely from the trunk, and swathed in a 

 common sheath of skin, they present an interesting ana- 

 logy to the embryonal buds of the answerable members 



