KUTCHIN. — PERIPHERAL NERVOUS SYSTEM OF AMPHIOXUS. 605 



along the dorsal median line, and laid open with the internal surfaces 

 uppermost, proved very interesting when studied with the aid of 

 strong light. Figure 32 (PI. 6) is drawn from a specimen dissected 

 after this fashion. It is e^'ident from this figure that many more 

 nerve branches penetrate to the interior than can be observed from an 

 external view. These branches appear smooth and darkly stained 

 for a short distance after they reach the interior, and their finer 

 branches ramifying over the base of the ventral fin have a similar 

 appearance. As the visceral branches ascend and ramify on the 

 internal surfaces of the muscles, they are more lightly colored, and 

 often flatten out into comparatively broad bapds, which appear 

 loosely confined. On the right side may be noted branches which 

 probably supply the posterior extension of the atrium. The nerves 

 raniifying over the base of the ventral fin are not to be confused with 

 the cutaneous supply. The latter may be seen in a specimen dissected 

 and examined in the manner just described, but they lie at a much 

 lower focus (i. e. nearer the surface). A few nerves were observed 

 forming a network OA'er the inner surface of the side muscles posterior 

 to the anus, thus accounting for at least a part of the visceral nerves 

 observed from the exterior in this region. The nerve supply for the 

 intestine probably reaches the latter tlirough the membrane (mesen- 

 tery) which holds it in place beneath the notochord. This corre- 

 sponds to the manner in which a nerve supply reaches the pharynx. 

 The presence of the post-atrial extension on the right side of the body 

 in this region, may bring about modifications in the arrangement of 

 the visceral nerves of that side. Certain abruptly ending branches, 

 indicated at c, c, in Figure 32, may be the result of imperfect impregna- 

 tion, or have a significance not yet determined. It seems certain 

 that a netw^ork of nerve threads similar to that noted in the branchial 

 region spreads over the internal surfaces of the muscles posterior to 

 the atriopore. 



In certain transverse sections of the tail region, stained v.ith 

 INIallory's differential stain, a pathological condition was evident, in- 

 vohing nearly all the structures of one myomere, and present to a 

 slight extent in an adjoining one. 



Considerable variation was noted in the total number of dorsal 

 nerve pairs in each species. In Branchiostoma lanceolatum the 

 number counted most frequently was 63; while in B. caribaeum the 

 number appeared to be between 56 and 60. 



