ALASKA NEMERTEANS 55 



Proboscis. — The proboscis sheath is long and slender. Even in this 

 elongated species it reaches within a few millimeters of the posterior end 

 of the body. The proboscis also is slender, though of moderately large 

 size. It reaches well backward in the body. The armature of the 

 proboscis is especially remarkable. The central stylet is moderately 

 slender, rather small, and rests on a moderately heavy basis. The 

 basis is somewhat conical in form, and rounded posteriorly (pi. vii, fig. 

 5). In addition to the central stylet there are usually 8 pouches of ac- 

 cessory stylets (p1. vii, fig. 5), though the number varies from 6 to 

 12. In each pouch are one or two slender stylets. Often there is a 

 single fully developed stylet, and a second, immature stylet in most of 

 the pouches. The proboscis is usually extruded when the animal is 

 killed. 



Cerebral sense organs. — Situated far in front of brain — nearly at 

 end of snout when the head is contracted — and fairly well developed. 

 Their canals open on antero-lateral borders of tip of snout. 



The blood-vascular system resembles that in other species of the 

 genus. 



Nefhridia. — The nephridial system shows peculiar deviations from 

 the arrangement usually found in the Metanemerteans. A pair of 

 main canals with numerous branches runs longitudinally above the 

 lateral nerve cords, as in other species. These reach forward well 

 toward the brain and extend posteriorly far into the intestinal region. 

 Their branches ramify both above and below the lateral nerve cords. 

 The number and position of the efferent ducts is remarkable — there are 

 commonly 20 or more on each side. The first is near the anterior end 

 of the main nephridial canal, and sometimes opens on the ventro-lateral 

 aspect of the body, as in other species of the genus. Back of this, 

 however, were counted nine other efferent ducts on each side in the 

 esophagal region, and at least as many more were present on each side 

 in the intestinal region. These ducts were mostly small and opened 

 on the dorsal aspect of the body, as in many Heteronemerteans. The 

 positions of the efferent ducts were sometimes immediately above the 

 lateral nerves, and sometimes but a little laterally from the proboscis 

 sheath. Most commonly, however, the ducts occupied positions be- 

 tween these two extremes, so that the majority of the nephridiopores 

 were situated on the dorsal surface about half way between the lateral 

 margin and the median line (p1. xi, fig. 3). As noted on page 52, an 

 approach to this condition is found in A. leuciodus. These appear to 

 be the only species of the genus, and indeed the only Metanemerteans, 

 in which the nephridiopores are situated on the dorsal surface of the 

 body. 



