70 



JiOOLOGY 



SECT. XIII 



Beneath the skin comes the muscular layer. This is always 

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

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

 segments or myomeres (Fig. 759, mym.), separated from one 



another by partitions of con- 

 nective-tissue, or mynrommas 

 (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 vertical jjlane 

 outwards 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. on.) portion. In the 

 higher grouj)s this segmental 

 arrangement, though present in 

 the embryo, is lost in the adult, 

 the mj'omeres becoming con- 

 verted into more or less longi- 

 tudinal bands having an ex- 

 tremely complex arrangement. 

 In the trunk, as shov/n b}' 

 a section of that region, the 

 muscles form a definite layer 

 beneath the skin and enclosing 

 the cwlome (Fig. 760, A and C, 

 cceL). 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 by a canal, the 

 neural or cerchro-sjnunl cavity 

 (c. s. c), 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), the ccclome is absent in the 

 adult, and the muscles occup\- practically the whole of the interval 



