344 MANUAL OF ELEMENTARY ZOOLOGY 



each is bent four times, so that it runs a zigzag course from 



the middle of the back to that of the belly. 

 Mwemmts? In the muscles of the head, throat, and fins the 



segmental arrangement is not apparent. The 

 myomeres are separated by partitions of connective tissue 

 (myocommata), between which their fibres run longitudinally. 

 By the action of these muscles, especially in the powerful tail, 

 which is more than half the length of the body, the animal 

 is driven through the water, the tail working from side to 

 side with a twisting motion, as an oar may be used from 

 the stern of a boat to propel it. The upturned position of 

 the axis of the tail helps to keep the snout upon the ground 

 as the fish " noses " about for its food. The dorsal and 

 ventral fins act like the keel of a boat in keeping the body 

 upright, and the paired fins help in balancing and are used 

 in steering upwards and downwards. 



The scales of the dogfish constitute a part of the 



skeleton known as the exoskeleton, which in 

 skeleton: the frog is represented only by the teeth. 

 Features. The endoskeleton of the dogfish corresponds to 



that of the frog in its main outlines, but differs 

 from it in some important respects, (i) It is wholly 

 cartilaginous, like that of the tadpole, containing nothing 

 which corresponds either to the membrane bones or to the 

 cartilage bones of the frog, though in places the cartilage is 

 calcified. (2) The axial skeleton is traversed below the 

 central nervous system by a peculiar rod, the notochord, 

 which consists of large, vacuolated cells with stout walls, 

 and is derived from the endoderm in the course of develop- 

 ment. This is present in the tadpole, but in the adult frog 

 is represented only by pads of tissue between the centra of 

 the vertebrae. (3) There are no structures which represent 

 any part of the sternum. (4) In correspondence with the 

 difference in the shape of the limbs, their skeleton differs 

 entirely in the two animals. (5) Unlike that of the 

 tadpole, the median fins are supported by rays. 



The backbone consists of about 130 vertebrae, in each 



Baokhono of wnich 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 



