KUTCHIN. — PERIPHERAL NERVOUS SYSTEM OF AMPHIOXUS. 581 



possesses a dorsal and a ventral ramus, although in rare cases either 

 may be absent, with the possible exception of the ventral ramus of the 

 left side. I was unable to find any specimens in which the latter 

 branch was entirely lacking. Occasionally the root subdivides at or 

 near its place of exit from the neural tube, and either ramus is repre- 

 sented by two or more branches. The primary dixision in nerve III 

 usually occurs near the neural tube, but considerable variation exists 

 in this regard. Different forms of division in the two species are 

 illustrated in Figures 6, 7, 11 and 12. In these figures the compara- 

 tive uniformity in the place of exit of the nerve through the myoseptum 

 may be noted. 



The curious post embryonic history of the buccal region brings 

 about the well known asymmetry in certain of the buccal nerve pairs. 

 The dorsal ramus of the third nerve of either side usually innervates 

 the neighboring portion of the dorsal fin, and a small area overlying 

 the adjoining trunk muscles. A small branch of the ventral ramus of 

 nerve III on either side of the body supplies the posterior ventral 

 portion of the rostrum. Twigs from this branch may anastomose 

 with branchlets of nerve II. In its course over the side muscles, this 

 more anterior ventral branch of nerve III gives off numerous sub- 

 dividing side branches. The main branch of the ventral ramus of 

 either side takes a course ventrad and noticeably anteriad over the 

 side muscles, and also gives off a number of side branches destined 

 for the skin of this region. In this part of its course, or at the ventral 

 border of the trunk muscles, this main branch of the ventral ramus of 

 the third nerve, on either side of the body, gives off a branch (or 

 branches) which passes to the anterior portion of the border of the 

 mouth of the corresponding side and breaks up into the outer mouth 

 plexus of this region. This branch usually leaves the more ventral 

 side of the main branch, but may vary considerably in size and posi- 

 tion. The main ventral branch of left nerve III is usually of greater 

 size than the corresponding branch of the right side, and supplies a 

 larger territory. This main branch divides at or near the ventral 

 border of the trunk muscles of the left side into two branches, one of 

 which passes beneath the ventral border of these muscles and emerges 

 on the right side of the body, where it is continued across the oral 

 hood to form the anterior portion of the inner mouth plexus of this 

 side. The other branch traverses the oral hood on the left side, and 

 breaks up into the left anterior portion of the inner mouth plexus. 

 Thus the most anterior portion of this inner plexus on either side of the 

 hood, is supplied by branches from left nerve III, while the outer mouth 



