KUTCHIN.— PERIPHERAL NERVOUS SYSTEM OF AMPHIOXUS. 583 



tions, the place of division in the first 6-8 anterior ner\-es may occur 

 at, or sHghtly after, the exit of the nerve from the myoseptum, al- 

 though it frequently occurs nearer the neural tube. 



The stem of the dorsal ramus of ner^^e IV is usually deflected more 

 or less anteriorly (Figs. 6 and 7, PI. 2). This ramus divides repeatedly, 

 distributing its branchlets o\'er the surface of the neighboring trunk 

 muscles, and in that portion of the dorsal fin lying approximately 

 dorsal to the nerve root. The ventral ramus of nerve IV is usually 

 of greater size than the corresponding ramus of nerve III, especially 

 upon the left side. This ramus of nerve IV takes a general ventrad 

 com'se over the trunk muscles; but in its course over these muscles 

 it forms an arch the convexity of which is directed anteriorly. This 

 arch is frequently more prominent on the left side, and a like one may 

 also be noted in the ventral ramus of nerve V (PI. 2, Fig. 7). Dogiel 

 ( : 02) states that an arch is also formed in the course of the ventral 

 ramus of nerve III, and occasionally in nerve VI. I, however, have 

 observed a marked arch only in the course of nerves IV and \. On 

 the left side this arch is accentuated by the fact that the branch of this 

 ramus which innervates internal substructures bends toward the in- 

 terior of the body. The ventral ramus of nerve IV gives off numerous 

 subdividing branches in its course over the side muscles. On the right 

 side it divides into two or more descending branches, which innervate 

 a portion of the outer wall of the oral hood, and finally break up into 

 a part of the outer mouth plexus. These descending branches may 

 be formed either in the course of the A'cntral ramus over the trunk 

 muscles, or at the ventral border of these muscles, or even in the oral 

 hood. The branches which cross the oral hood usually gi\-e off only 

 small side branchlets. The ventral ramus of left nerve IV innervates 

 a much larger territory, and shows considerable variation in the num- 

 ber of its branches, and their manner of distribution. This ramus 

 usually di\'ides into its main branches near the ventral border of the 

 side muscles, but often varies in this regard. A branch, or branches, 

 to the outer plexus of the mouth may be given off from the main 

 branch at any point in its course over the trunk muscles (Fig. 7). 

 Other branches to this plexus may be given off at or near the place of 

 main division mentioned above. 



The remaining branches formed at this place of division are destined 

 for less superficial structures. Of these, one or more take a course 

 ventrad across the oral hood and form that part of the inner plexus of 

 the mouth lying posterior to the portion formed by branches of left 

 nerve III. The branches of left nerve IV which aid in the formation 



