70 



'/J )OL()OY 



SECT. XIII 



Beneath the skin cimies the muscular layer. This is always 

 higlily developed, and, in the lower Craniata, has the same general 

 arrangement as in Amphioxns, i.e., consists of zig-zag muscle- 

 segments or iiti/oiitcrcs (Fig. 759, mym.), separated from one 



another by partitions of con- 

 nective-tissue, or myoeommas 

 (myc), and formed of longitu- 

 dinally disposed muscle-fibres. 

 The myomeres are not placed 

 at right-angles to the long axis 

 of the body, but are directed 

 from the median vei'tical jjlane 

 outWfirds and backwards, and 

 are at the same time convex in 

 front and concave behind, so 

 as to have a cone-in-cone ar- 

 rangement (Fig. 760, C). Each 

 myomere, moreover, is divisible 

 into a dorsal {d. m.) and a ven- 

 tral (v. m.) portion. In the 

 higher groups this segmental 

 arrangement, though jjresent in 

 the embryo, is lost in the adult, 

 the m3romeres becoming con- 

 verted into more or less longi- 

 tudinal bands having an ex- 

 tremely complex arrangement. 

 In the trunk, as shown by 

 a section of that region, the 

 muscles form a definite layer 

 beneath the skin and enclosing 

 the cozlomc (Fig. 760, A and C, 

 ixcl.). The muscular layer, as 

 in Amphioxus, is not of even 

 diameter throughout, but is 

 greatly thickened dorsally, so 

 that the coelome is, as it were, 

 thrown towards the ventral 

 side. Its dorsal portion, more- 

 over, is excavated b}' a canal, the 

 neural or cerchro-qnncd cavity 

 (e. s. r.), in which the central 

 nervous system is contained, and the anterior portion of which is 

 always dilated, as the cranial cavity, for the brain. Thus a 

 transverse section of the trunk has the form of a double tube. 

 In the head, neck, and tail (B, D), tlie ccolome is absent in the 

 adult, and the muscles occu])y praoticall}' the whole of the interval 



