KUTCHIN. — PERIPHERAL NERVOUS SYSTEM OF AMPHIOXUS. 585 



latum), however, after taking a posteriorly directed course for some 

 distance along the inner border of the trunk muscles of the left side, 

 this branch turned dorsad running toward and a little beyond the 

 median plane, where it entered the velum slightly to the right of this 

 plane. This is the only instance observed of a left nerve branch 

 entering the velum on the right side of the bod3^ In some cases a 

 branch of left nerve IV is indirectly connected with the innervation of 

 the velum by anastomosis with a large branch of left nerve V which 

 passes directly to the velum. These variations occur in both species, 

 and make it difficult to define the territory innervated by the fourth 

 nerve pair. Such variations no doubt account, in large measure, for 

 the apparent disagreements between the various descriptions of this 

 nerve. 



N^erve V. — Nerve V (PI. 2, Fig. 7) is in many respects similar to 

 nerve IV. It is usually of about the same size, and the ventral ramus 

 forms an arch similar to that described for nerve IV. The dorsal 

 ramus of nerve V innervates a territory corresponding to that of the 

 dorsal ramus of nerve IV, but more posterior, and the portion of the 

 ventral ramus taking a course over the trunk muscles gives off branches 

 similar to those leaving this portion of the ventral ramus of nerve IV. 

 The place of division of nerve V into a dorsal and a ventral ramus in 

 B. caribaeum is illustrated in Figure 6. The fifth nerve pair is also 

 bilaterally asymmetrical. 



The ventral ramus of right nerve V usually divides at or near the 

 ventral border of the trunk muscles into two or more branches, which 

 cross the oral hood and form that part of the outer mouth plexus lying 

 posterior to the portion formed from branches of right nerve IV. 

 Occasionally only a single small branch of right nerve V joins the outer 

 plexus of the mouth border. The ventral ramus of left nerve V is 

 larger than that of right nerve V, and exhibits considerable variation 

 in its manner of division. The main branches are usually given off 

 at or near the ventral border of the lateral muscle. In Figure 7 (B; 

 caribaeum) a branch to the inner mouth plexus may be observed 

 leaving the ventral ramus about half way in its course over the trunk 

 muscles. One or more branches usually pass to the outer mouth 

 plexus from the main place of division of this ventral ramus, and a 

 like number leave this place for the inner plexus of the mouth border. 

 A band-like nerve branch is also given off from left nerve V near the 

 ventral border of the side muscles, which frequently anastomoses 

 with a branch of left nerve VI (Figure 8). This nerve branch is usually 

 concerned in the innervation of the velum. Another form of con- 



