192 COMPARATIVE ANATOMY. 



anterior portions of the body, owing to its relations to the branchial 

 skeleton. 



In the Cyclostomata also the greater part of the muscular 

 system has no direct connection with the skeleton, for the superficial 

 layers are here also merely connected with connective tissue, and the 

 septa that form the metameres are to be found over the whole of the 

 dorsal and caudal regions of the body. On the head, however, and 

 on the visceral skeleton, we may see that some of the muscles 

 are connected with the skeletal parts and differentiated in a special 

 manner. 



When the skeleton is formed it is necessarily connected with 

 the musculature, since the skeletal parts grow down between the 

 masses of muscles, following the septa of connective tissue. The 

 primitive similarity, therefore, between the parts of the mus- 

 cular system disappears, and a differentiation commences; this is 

 implied, on the one hand, by the formation of a muscular system 

 connected with the skeleton, and on the other by the special 

 development of the remainder of the muscular system, which is 

 not so connected, into a dermal musculature. 



The whole of the muscular system requires, however, to be 

 systematically investigated before we can know as much about it as 

 we do about the skeleton. We must, therefore, confine ourselves in 

 this description to a mere sketch, many details in which must be put 

 in with great diffidence. 



Dermal Muscles. 



§ 371. 



As we regard the dermal muscles as primitively forming a 

 common complex with those which belong to the skeleton, we must 

 distinguish from it those which belong to the integument as such. 



Among the Cyclostomata some of the muscles of the trunk have 

 no connection with the parts of the skeleton, and appear, therefore, 

 to be essentially dermal muscles ; even in the lower Gnathostomata, 

 the greater portion of the large lateral masses of muscles on the trunk 

 are only connected to the skeleton by the tendinous intermediate 

 bands, which pass off from it ; it has not, therefore, yet become 

 a part of the skeletal musculature, in the sense that it forms bundles 

 of muscles which are attached to the skeleton by their two ends of 

 origin and insertion. This more indifferent condition enables us 

 to understand how it is that there are no distinct dermal muscles. 

 At the same time there are distinct layers of dermal muscles in the 

 outer wall, at least, of the respiratory cavity in the Selachii, where 

 they form part of a common constrictor. 



In many other parts also of the body there are subcutaneous 

 muscles, which are not connected with the large lateral muscles; the 

 layer which runs along the lateral line in the Teleostei, and Avhich is 



