POLYCHEZTA FROM THE N.E. PACIFIC. 959 
International Boundary. It is thus widely distributed in the 
neighbourhood of Puget Sound and the Gulf of Georgia. 
One whole specimen was obtained with the following dimen- 
sions :— 
Length 28:5 em, Width in broadest part 1 em. 
Anterior region with 9 segments 1°8 em. long. 
Median region with 3 segments 4:0 cm. 
Posterior region with 68 segments 22°7 cm. 
This was probably a small individual, since in others measured 
the anterior and median regions exceeded the figures given 
above. 
Mesochetopterus is a longer and much more slender worm than 
Chetopterus variopedatus. The delicacy and transparency of the 
integument, which is so marked a feature of the last-named 
species, is not characteristic of the new genus. The whole 
surface is a creamy white or yellow, relieved by dashes of choco- 
late pigment on the peristomium and tentacles alone. In the 
posterior region the dark green gut shows through the body-wall. 
The ventral musculature is, throughout the body, more developed 
than in Chetopterus. 
The prostomium (PI. I. fig. 3) is a small rounded prominence 
with unpigmented skin. Itis much more distinct than in Cheto- 
pterus. There is no trace of eyes. The peristomiwm forms a 
prominent buccal funnel which entirely surrounds the prostomium. 
It is, however, shallower, with more gently sloping sides than 
in Chetopterus. As mentioned above, there is here a noticeable 
development of the chocolate pigment, which does not dissolve in 
alcohol, and so is retained by the preserved specimens. The 
peristomial tentacles originate just outside the peristomial collar. 
They are stout grooved structures (very extensile in the living 
animal) and measuring, even in their contracted state, 3-4 em. 
They contain a pigment similar to that mentioned above. The 
mouth is situated between the prostomium and the ventral lip 
of the peristomium, and is bordered in some specimens by two 
rounded lips below. 
The anterior region is convex on the ventral side, concave on 
the dorsal. It differs, however, generally from that of Chatopterus, 
firstly in the narrower width and secondly in the shortness of the 
notopodia. In 21 specimens examined, 10 had 9 segments, an 
equal number had 10 segments, and a single individual 11. In 
Chetopterus variopedatus, similarly, though 9 is the typical 
number, Joyeux Laffuie observed indiv iduals with 10, 11, and 
even, in one case, 12 segments. The parapodia of the anterior 
region are all similar and represented by the conical notopodia, 
which increase slightly in size as we pass posteriorly. The 
dorsal curvature is not so distinct as in Chetopterus. ‘The last 
segment does not bear an appendage representing the neuro- 
podium, such as occurs in C. variopedatus (Joyeux Laftuie, 9, 
p. 257, pl. xv. fig. 2) and other species. 
