MUSCLES AND MOVEMENTS 31c 



3 and 8 in Fig. 238). Since the lateral components of the force 

 which the moving tail exerts on the water are alternated and 

 cancel each other out, the resultant is a force directed backwards. 

 The equal and opposite force exerted, in accordance with Newton's 

 first law, by the water on the fish, drives the fish forward. The tail 

 fin (though in this respect less important than that of most fishes) 

 adds to the propellant surface which is applied to the water when 

 a wave reaches the end of the body. To produce a turn, a strong 

 contraction is sent down one side and turns the head to that side. 

 The tail, owing to the resistance offered by its fin, stands firm as a 

 fulcrum for the head-turn ; afterwards it is swung into line with 

 the head. The unpaired fins act like the keel of a boat in reducing 

 rolling and yawdng, while the heterocercal tail, acting in con- 

 junction with the pectoral fins acting as elevators, enables the 

 fish to alter its level in the water. 



SKELETON I GENERAL FEATURES 



The endoskeleton of the dogfish corresponds to that of the 

 rabbit in its main outlines, but differs from it in some important 

 respects, (i) It is wholly cartilaginous, containing nothing which 

 corresponds either to the membrane bones or to the cartilage 

 bones of the rabbit, though in places the cartilage is calcified. 



(2) The axial skeleton (pp. 318-26) is traversed longitudinally be- 

 low the central nervous system by a peculiar rod, the noto- 

 chord, which consists of large vacuolated cells with stout walls, 

 and is derived, in the course of development, from the roof 

 of the primitive alimentary canaL A notochord is present in 

 the young of all vertebrates, but disappears in most adults. 



(3) There are no structures which represent any part of the 

 breast bone. (4) In correspondence with the difference in form 

 of the limbs there are large differences between their skeletons. 

 (5) The m9dian fins are supported by rays. 



BACKBONE 



The backbone consists of about 130 vertebrae (p. 422), in each 

 of which the centrum is pierced from end to end by a canal for 

 the notochord. This canal is narrower in the middle of the vertebra, 

 so that the notochord is constricted, and after its removal the 

 centrum appears as a biconcave disc. On each side the centrum 



