ALASKA NEMERTEANS 73 



the circular muscles of the esophagus increase so greatly in number 

 that they form a most conspicuous layer. In the region of its maximum 

 development this layer becomes nearly half as thick as the circular 

 layer of the body walls in the same section. In no other species of 

 the Lineidce has this muscle been found of even approximately this 

 thickness. Its fibei-s connect in part v^ith the circular layer of the body 

 walls, and to a lesser degree with the circular muscles of the proboscis 

 sheath. But few fibers lie on the dorsal wall of the esophagus, so 

 that this organ is largely bound up with the proboscis sheath in a con- 

 tinuous layer of muscles, and one cannot fail to see the striking resem- 

 blance between this circular layer and the inner circular muscles that are 

 so highly developed in precisely the same region in Carinoma (p. 22). 



Bcfdy "Walls. — The external longitudinal muscular layer of the body 

 walls is especially weak when compared with the same layer in related 

 species. This is partly shown by the fact that the cutis glands extend 

 Y^ or more of the distance from the exterior to the circular muscular 

 layer. Nearer the intestinal region the muscular layers are thicker, 

 and the esophagus occupies correspondingly less space. 



The three longitudinal blood vessels are very conspicuous through- 

 out their length. The dorsal vessel leaves the proboscis sheath near 

 the posterior ends of the nephridia, or at about ^ the distance towards 

 the posterior end of the esophagal region. 



Nephridia. — The nephridial system consists of a pair of unusually 

 • large, longitudinal canals, which lie in the dorsal walls of the lateral 

 blood lacunae beside the esophagus. The nephridial canals send off 

 very few branches, except near their anterior ends, where they divide 

 into smaller branches. The main canals extend through about the 

 middle third of the esophagal region. Each of the pair of longitudinal 

 canals terminates posteriorly in a single, remarkably large efferent 

 duct which opens on the dorso-lateral aspect of the body. The ducts 

 sometimes lie exactly opposite, but in other specimens one lies some 

 distance farther back than the other. In such cases, of course, one of 

 the nephridia extends a corresponding distance farther posteriorly than 

 the other. Each of the efferent ducts spreads out as it passes through 

 the circular muscular layer into a broad sieve-like or filter-like struc- 

 ture with scores of small, nucleated cells. From this point a straight 

 and comparatively narrow duct leads directly to the surface. 



Reproductive glands. — The sexual elements are fully mature in the 

 month of June, and the genital ducts at this time are fully formed. 

 In the females each duct communicates directly with the cavity of the 

 ovary by a funnel-shaped opening, and a similar funnel-shaped open- 



