680 PROCEEDINGS OF THE ACADEMY OF [DeC. , 



dorsal nerve, first termed by Hubrecht the " Riisselsclieidennerv, " 

 later the "medullary nerve;" by Biirger denominated the " oberen 

 Riickennerv. " The dorsal nerve extends throughout the body 

 and ends about .06 mm, in front of the anal opening. No con- 

 nection could be traced between the end of the dorsal nerve and 

 the lateral nerves, but it is probable that a delicate plexus exists. 

 The position of the dorsal nerve is constant, just above the circu- 

 lar muscle in the median plane of the body. A second doi*sal 

 nerve (" Riisselscheidenucrv," Hubrecht; " unterer Riickennerv," 

 Burger), separates off from the first shortly after its origin, passes 

 inside the circular muscle of the body wall and lies just above the 

 proboscis sheath. Both nerves are of fair size, about .017 mm. 

 in diameter in the anterior part of the body, but both become 

 much enlarged in the region of the muscular crosses between the 

 greatly thickened inner circular muscle and that of the body wall, 

 Plate XLI, fig. 23. Here for a short distance the diameter of the 

 inner nerve reaches a thickness of .03-. 06 mm. There is consider- 

 able variation in the respective sizes of the two nerves in different 

 specimens. Sometimes the upper nerve is the larger one, but more 

 often the lower nerve attains the greate" size. The greatest size 

 observed was in a specimen whose lower dorsal nerve measured 

 .117 ram. dorso-ventrally and .058 mm. across; the upper dorsal 

 nerve measuring .017 mm. iu both directions. This enlarged con- 

 dition coincided with the thickest part of the inner circular muscle 

 layer. The same coincidence has been noted in the Protonemerteans 

 and OaH/io?Ha by Hubrecht (1887), who says, p. 80: '' The fact that 

 in this (oesophageal) region of Carinoma the proboscidian sheath- 

 nerve comes into the foreground so strongly that it might easily be 

 mistaken for the medulla, may probably be ascribed to the mas- 

 sive development of the inuer circular muscular layer 8, which iu 

 Carinina, Carinella and Carinoma acts at the same time as part of 

 the wall of the proboscidian sheath. The fact was already noticed 

 as a peculiar feature of the species by INI'Intosh (1875), when he 

 first described Carinoma (under the name of Valenciniaarmaiidi)." 

 Posterior to the region of the iuner circular muscle the two dorsal 

 nerves resume their normal size, and then gradually decrease until 

 they terminate near the end of the body. 



Throughout the greater part of the body the lateral nerves are 

 connected with each other and with the upper dorsal nerve by a 



