ZOOLOGY 



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 Amphioxus, i.e., consists of zig-zag muscle- 

 segments or myomcres (Fig. "759, .myin.), separated from one 



another by partitions of con- 

 nective-tissue, or myocommas 

 (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 plane 

 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. m.) portion. In the 

 higher groups this segmental 

 arrangement, though present in 

 the embryo, is lost in the adult, 

 the myomeres 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 ccelome (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 coalome is, as it were, 

 thrown towards the ventral 

 side. Its dorsal portion, more- 

 over, is excavated by a canal, the 

 neural or cerebro-spinal 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 ccelome is absent in the 

 adult, and the muscles occupy practically the whole of the interval 



