THE CCELOM AND MESODERM 277 



Mention must be made of the fact that in some cases the 

 electric difference of potential which always accompanies 

 muscular activity has been specially increased, with the result 

 that some muscles have been converted into " electric organs." 

 It is interesting to notice that while in Raia it is the somatic 

 (myotomic) muscles in the region of the tail that have become 

 thus modified, in Torpedo it is the visceral muscles derived 

 from the visceral arches. 



The first three pairs of somites (in the Craniates) are small 

 and give rise to the extrinsic eye-muscles. From the fact that 

 they are situated in front of the ear, they are known as prootic 

 somites, and their development is described in Chapter XXVIII. 

 The myotomes which are produced from the next posterior 

 (or metotic) somites are divided by the gill-slits into dorsal 

 and ventral portions, the latter portion forming the hypoglossal 

 muscles. 



In Amphioxus, each myotome is a plate of muscle extending 

 from near the middorsal to the midventral line, on one side of 

 the body. When seen from the side, each myotome is bent 

 into the shape of a V with the apex pointing forwards. In 

 Petromyzon, the myotomes behind the region of the gill-slits 

 are like those of Amphioxus ; only the septa are slightly more 

 bent so that each myotome seen from the side is in the form of 

 a W. In fish and all higher forms, however, each myotome 

 behind the gill-slits is divided into two by a horizontal parti- 

 tion or septum. It is in this septum that the " true " 

 or dorsal ribs are formed. The myotomes are then repre- 

 sented by dorsal or epaxonic, and by ventral or hypaxonic 

 muscles. 



The muscles of the fins in Craniates are formed from 

 " muscle-buds," which are nipped off from the myotomes. 



In the Tetrapods, the epaxonic muscles are much reduced, 

 while the hypaxonic muscles assume greater importance. The 

 muscles of the paired fins and limbs are derived from the 

 hypaxonic portions of the myotomes. Apart from the latter, 

 the great development of which in Tetrapods is connected 

 with the greater strength necessary for locomotion on dry land, 

 the hypaxonic portions of the myotomes also give rise to the 



