INTERNAL ANATOMY. 35 



in correlation with this condition is to be noted the absence 

 of a distinct sympathetic nervous system. 



The striated muscles can be arranged in two groups : 

 (i.) the parietal muscles constituting the myotomes or 

 muscular segments of the body, and (ii.) the visceral 

 muscles which arise independently of the myotomes and 

 are not segmentally arranged. The smooth muscle-fibres, 

 which occur on the walls of the alimentary canal and 

 blood-vessels, may be grouped together as the splanchnic 

 muscles. 



The parietal muscles are the great longitudinal muscles 

 which make up the thick lateral walls of the body. In 

 Amphioxus they form collectively the essential organ of 

 locomotion. The portion of them lying next to the atrium 

 on each side, and stretching from the notochord to the 

 base of the myotome, is placed at an angle to the rest, and 

 has a more vertical direction. (Cf. Fig. 2.) This has 

 been described by SCHNEIDER as the rcctus abdomiuis. It 

 probably co-operates with the muscles of the floor of the 

 atrium to cause the contraction of the latter cavity for the 

 purpose either of expelling water or reproductive elements 

 through the atriopore. 



The visceral muscles consist of (a) the transverse muscles 

 stretching across the floor of the atrium (cf. Fig. 2), (/3) 

 muscles of the oral hood and cirri, (7) sphincter muscle of 

 the velum (cf. Fig. 7), (>) anal sphincter. 



All the striated muscles of Amphioxus are composed 

 of highly characteristic flat lamelliform plates, which can 

 often be resolved into a great number of finer fibrils. In 

 the longitudinal muscles of the adult, nuclei are very 

 rarely met with, but in other places they are to be found ; 

 as, for instance, in the fibres composing the velar sphincter 

 (Fig. 1 6). 



