ALASKA NEMERTEANS 



29 



basis of the central stylet is massive and has a concave or a truncated 

 posterior end, while the stylet itself is comparatively weak. Of course 

 the number of nerves in the proboscis cannot be considered a generic 

 character. There are two Alaska species, then, which may be placed 

 in the genus — Z. thalassina^ which is closely allied to the type species, 

 and Z. albida, which is a minute, white form. 



8. ZYGONEMERTES THALASSINA sp. nov. 

 p1. II, fig. 5 ; p1. VII, fig. I ; p1. xiii, fig. 2. 



Zygonemertes thalassina differs from Z. virescens chiefly in the 

 following peculiarities : Color of former species much darker ; eyes 

 more numerous ; shape of basis and central stylet of proboscis differ- 

 ent ; usually 5 stubby stylets in each lateral pouch, and 12 nerves in 

 proboscis. There are minor differences in other anatomical details. 



Zygonemertes thalassina has a slender, somewhat flattened body ; 

 head broad, not sharply marked off from body ; one or two pairs of 

 very inconspicuous oblique furrows on sides 

 of head. The worms are active, and are rest- 

 less in confinement. 



Ocelli. — Ocelli very numerous ; arranged 

 in two or three longitudinal rows along sides 

 of head, and extending backward along lat- 

 eral nerve cords far behind brain. They are 

 smaller posteriorly than in front and are more 

 widely scattered. In front of the brain there 

 are sometimes 40 or more ocelli arranged in 

 two or three irregular rows (fig. 5) which 

 follow the general outline of the lateral mar- 

 gin of the head. Immediately in front of the 

 brain about a half dozen much smaller eyes 

 are seen, and lateral to the brain are com- 

 monly 10 to 12 ocelli of moderate size. Be- 

 hind these are usually 8 to 15 small ocelli 

 scattered along the nerve cords at irregular 

 intervals. These commonly reach nearly as 

 far behind the brain as the distance from the 

 brain to the tip of the snout in moderate ex- 

 tension. In sections (p1. xiii, fig. 2) they 

 are found to lie quite internal to the body musculature and almost di- 

 rectly upon the lateral nerves. 



Size and color. — The specimens obtained were from 30 to 60 mm. 



Fig. 5. Zygonemertes 

 thalassina. Dorsal view 

 of head showing outline 

 of brain and lateral nerves, 

 and arrangement of ocelli. 

 X12. 



