588 PROCEEDINGS OF THE AMERICAN ACADEMY. 



be noted in Figure 6 that this nerve leaves the neural tube very near 

 its dorsal surface, and arches noticeably immediately after its exit. 

 The dorsal ramus of nerve VIII on either side of the body, and the 

 branches of this nerve pair in the region of the trunk muscles, are so 

 similar to the corresponding branches of the two next anterior nerves, 

 already described, that a separate description is unnecessary. I have 

 been unable to find a branch of the ventral ramus of right nerve VIII 

 which unc^uestionably joins the outer plexus of the mouth border. 

 However, since branches of this nerve sometimes lie considerably 

 anterior to the velum, it seems probable that they may occasionally 

 contribute toward the formation of that plexus. Left nerve VIII 

 frequently sends a small branch to this plexus (Fig. 7), but such a 

 branch is as frequently lacking. None of the specimens examined 

 showed a branch of left nerve VIII passing to the velum, as described 

 by Dogiel (:02). 



The variation in the territory innervated by the indi\adual anterior 

 nerves is of considerable interest, since the branches of a nerve usually 

 supplying a certain area, may be supplanted by those of another 

 nerve which leaves the neural tube at some distance from the place of 

 exit of the nerve ordinarily innervating the given territory. It ap- 

 pears from this that the same nerve does not always carry fibers to 

 precisely the same area. The plexuses and communicating nerve 

 bands of this region may have a bearing on this point, and lead to 

 doubt as to which nerves actually supply certain territories. The 

 relation of the nerve components of the neural tube to each other in 

 this region must also be of interest. It is certainly a fact worthy of 

 consideration that a great nerve, like nerve IV, leaving the neural 

 tube at a definite place, may or may not take a prominent part in the 

 innervation of the velum. 



The plexuses of the mouth border have been frequently described, 

 especially by Fusari ('89) and Dogiel (: 02), and will not be taken up 

 in detail in this paper. Besides the inner and outer plexuses of this 

 region, Dogiel describes a " Zwischengeflecht," l;yang between them 

 and taking its origin chiefly from the outer plexus. The nerve 

 branches taking part in the formation of the plexuses of the mouth 

 border in the specimens of both species observed by me have been 

 already enumerated. Figure 18 (PI. 4) illustrates a portion of the 

 outer plexus in Branchiostoma caribaeum. This plexus does not 

 appear to be essentially different from the corresponding plexus in 

 B. lanceolatum. The inner plexus is also apparently similar in the 

 two species. 



