THE ELASMOBRANCH FISHES 209 



muscle-segments or myotomes. The myotomes, however, are 

 at once complicated and more numerous than in Amphioxus. 

 There are thirty-four pre-anal and as many as ninety-six post-anal 

 segments, and each myotome instead of being bent once at an 

 angle is bent four times. Starting from the mid-dorsal line it 

 runs first forwards, then back at a sharp angle, then forwards 

 again as far as the lateral line. From the lateral line it runs 

 sharply backwards again and then bends forward under the 

 ventral surface to join its fellow of the opposite side. There 

 is also a specialisation of the musculature such as is not found 

 in Amphioxus. In the head the muscles are grouped about 

 the eyes and jaws in such a manner that all trace of metameric 

 arrangement is lost. In the neighbourhood of the gills the 

 muscles are modified in connection with the gill-pouches and 

 the cartilaginous skeleton supporting them ; there is a group of 

 powerful longitudinal muscles on the ventral wall of the throat, 

 and there are special muscles in connection with the paired 

 and median fins. It should be particularly noticed that the 

 primitive segmental arrangement of the muscles is lost in the 

 head, and largely modified in the region of the gills. 



The skeleton of the dogfish shows a great advance on that 

 of Amphioxus, but its main or axial portion is moulded upon 

 a continuous notochord, which in the embryo is very like that 

 of Amphioxus, except that it never extends to the anterior end 

 of the body. Considerable remnants of it are found in the 

 adult. The skeleton is divisible into the axial skeleton, 

 consisting of the cranium and vertebral column ; the visceral 

 skeleton, consisting of the jaws and branchial arches ; and the 

 appendicular skeleton, consisting of the cartilaginous supports 

 of the fins. The whole skeleton is composed of cartilage which 

 may be calcified in places, but no true bone is developed either 

 in substitution of the cartilage or in the membrane surrounding 

 the cartilage. 



It is convenient to begin with the study of the vertebral 

 column. This is made up of a number of vertebral bodies or 

 centra, bearing neural arches above and haemal arches below ; 

 the latter, however, are absent or modified in the trunk. The 

 vertebral centra are traversed by the notochord ; the neural 

 arches enclose a canal containing the spinal cord, and the 

 haemal arches of the tail enclose the caudal artery and 

 caudal vein. Each individual centrum is a short, stout 



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