576 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the anterior portion of the first myomere. Occasionally little of the 

 main stem of nerve I is visible; in other cases it may be observed for 

 almost its entire length, while in still other specimens its union with 

 the neural tube, and even a portion of the tube itself, are exposed to 

 view. The main stem usually lies close to the dorsal surface of the 

 chorda. 



Nerve II. — In 1841 Johannes Miiller described the second nerve, 

 enumerating it as the first (J. Miiller '44, p. 95, 96). Quatrefages 

 ('45) numbered the branches of nerve II as nerves II, III, IV, and V, 

 and figured the terminal branches of nerve I in the rostrum as arising 

 from nerve II. This conception was based upon his idea that the 

 first was an optic nerve, a short stalk connecting brain and "eye spot." 

 Schneider ('79) figures (Taf. 15, Fig. 1) two roots for nerve II of the 

 right side, stating (p. 14) that this nerve usually possesses two roots 

 upon one side, and upon the other divides immediately after its origin. 



Dogiel ( : 02) gives an extended and accurate account of the dis- 

 tribution of the branches of nerve II, as well as the variations which 

 occur in the form of the nerve itself, and the territory which it inner- 

 vates. He finds from one to three nerves in the region of the exit of 

 nerve II, conditions not being symmetrical in this respect on the two 

 sides of the body. He enumerates these irregularly occurring nerves 

 as distinct nerves, without considering their relation to the myomeres. 

 Although his figures do not present the muscle segments with clearness, 

 they convey an inaccurate idea of the number and position of these 

 segments. Dogiel describes several forms of variation from the usual 

 com-se to the rostrum of nerves I, II, and III (as he enumerates them). 

 In one case nerve II is poorly developed, and a branch of his nerve III 

 innervates the ventral part of the rostrum (Dogiel's Fig. Gh, which 

 illustrates this condition, shows the roots of nerves II and III in close 

 proximity). In another case, of frequent occurrence, nerve II is 

 strongly developed and only the posterior ventral part, if any, of the 

 rostrum receives branches of nerve III. (In Dogiel's Fig. 6c, and 6f/, 

 illustrating this case, the nerve designated as III is the one commonly 

 accepted as III.) Occasionally his nerve III (shown in his Fig. 7, 

 which is not the nerve usually enumerated as III) ramifies exclusively 

 in the skin of the dorsal fin, a ventral branch being absent. In such 

 cases the skin of the posterior ventral portion of the rostrum is sup- 

 plied by branches from his nerve IV. (From Dogiel's Fig. 7 this is 

 clearly the commonly accepted nerve III.) In another form of 

 variation (Dogiel's Fig. 8a, and 86) his nerve III consists of two or 

 three dorsal branches and a small ventral ramus, which does not reach 



