ZOOLOGY 



SECT. 



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

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

 segments or myomeres (Fig. 768, mym.), separated from one another 

 by partitions of connective- tissue, or myocommas (myc.), and formed 

 of longitudinally 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 arrangement (Fig. 

 769, C). Each my omere. moreover, 

 is divisible into a dorsal (d. m.) 

 and a ventral (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. 769, A and C, ccel). The 

 ^ i g- muscular layer, as in Amphioxus, 

 is not of even diameter throughout, 

 but is greatly thickened dorsally, 

 so that the ccelome is, as it were, 

 thrown towards the ventral side. 

 Its dorsal portion, moreover, 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 between the skin and 

 the skeleton, presently to be referred to : in the tail, however, there 

 is found a haemal canal (h. c.) containing connective-tissue, and 

 representing a virtual backward extension of the ccelome. The 



