594 PROCEEDINGS OF THE AMERICAN ACADEMY. 



nerve from the myoseptum (Fig. 6). In the remiiinder of this region 

 the place of division Hes deeper, (hie, in the main, to the greater 

 thickness of the trunk muscles; for the majority of the nerves of this 

 group divide at a comparatively uniform distance from the neural 

 tube. Much variation exists with regard to the place of exit of the 

 dorsal and ventral rami through the myoseptum (PI. 6, Fig. 34; PL 7, 

 Figs. 37, 39). Johnston ( : 05) states that "the typical place of 

 division is about half way between the cord and the dermis," rather 

 than close beneath the dermis as described by Hatschek. Johnston 

 finds much variation in the place of division, and frequent cases of 

 separate emergence of the dorsal and ventral rami. 



The dorsal rami of the nerves of this group resemble those of the 

 more anterior region. They are largest in the thickest portion of the 

 body, and possess a great number of branches, and, like the anterior 

 dorsal rami, innervate the cutaneous area over the neighboring trunk 

 muscles, and the adjoining portions of the dorsal fin. These dorsal 

 rami frequently emerge from the myoseptum not as single stems, but 

 in several branches, division having taken place beneath the dermis 

 (Figs. 34, 39). 



The ventral rami of these nerves are usually large, but may vary, 

 one or two rami being increased in size at the expense of an adjoining 

 ramus (Fig. 39). x\ ventral ramus may even be entirely absent. 

 Both dorsal and ventral rami g\\e off a munber of repeatedly di\-iding 

 branches in their course over the trunk muscles. In many methylene- 

 blue preparations which retain little epithelium, the finer branches 

 appear to end in innumerable delicate branchlets, the distal ends of 

 which are often noticeably directed outward. Frequently these 

 slender branchlets are continued into fine threads, which anastomose 

 with one another and with similar threads proceeding from adjoining 

 nerves, forming a delicate, irregularly meshed plexus. Such plexuses 

 are not limited to this region, but are also present in connection with 

 nerves posterior to the atriopore. These plexuses have been so little 

 observed and figured that a detailed account will be given of their 

 distribution and character in connection with the discussion of sensory 

 nerve endings. The ventral rami of the dorsal nerves of the branchial 

 region (with a few possible exceptions) di\ide near the ventral border 

 of the side muscles, to form three main branches (see sketch, PI. 4, 

 Fig. loa). This division is accomplished in various ways, and each 

 resulting branch may be represented by one or more twigs. The 

 most exterior branches formed by this division are the rami cutanei 

 vcntralcs, noted by early authors, and well described by Fusari ('89), 



