POLYDORA FLAVA. 207 



posteriorly from the intestine, and with a red blood-vessel in front and along the centre 

 of the dorsum, each gill also having a red vessel. 



The first foot is peculiar in having bristles in its dorsal division, which presents a 

 rounded lamella with a setigerous process in front (Plate XCVIII, figs. 16 and 16 a) 

 bearing a small group of tapering bristles slightly curved backward in situ. The ventral 

 division has a similar, but somewhat broader, lamella, in front of which is a conspicuous 

 tuft of longer upper and shorter lower bristles, the latter having distinct traces of 

 wings. The second (Plate XCVIII, fig. 16 b), third, and fourth feet have completely 

 formed dorsal and ventral divisions. The upper division consists of the long dorsal series 

 and a shorter anterior series (Mesnil) minutely punctated, and of a posterior series inter- 

 mediate in character, but also punctated. The ventral division has capillary winged 

 bristles in both rows, but at the seventh segment they are replaced by the winged hooks, 

 besides having in a few segments two capillary bristles inferiorly. 



The fifth segment (Plate C, fig. 10) has a minute group of dorsal and ventral 

 bristles as in other forms, and the great hook-like bristles (Plate CVI, fig 3 b) have the 

 concavity of their tips directed backward, and are arranged in a curve. In the ordinary 

 condition the shaft enlarges from the base upward to the middle, then slightly diminishes 

 to the tip, which forms a lever-like hook with a convexity posteriorly and an excavation 

 in front. The hook is more marked in some examples than in others. They are accom- 

 panied by a series of small spear-shaped bristles. In the developing hook the distal curve 

 is very marked, and a small shoulder appears at the base of the concavity in front, whilst 

 a lateral dimple and elevation, as well as a differentiation at the commencement of the 

 terminal curve posteriorly, are evident. In frontal view, as Mesnil shows, the distal 

 region is likewise differentiated. 



As a rule about six hooks are well developed on each side, and they form a 

 gradational series, commencing with one in which the blunt tip is scarcely curved and 

 ending with those in which the curvature at the sharp tip is marked. The developing 

 forms (two or three) have sharp, curved tips, but no trace of a spur is visible. They are 

 accompanied by bristles with peculiar elongate tips, the sides of which are nearly parallel, 

 and the tip in some is blunt. 



The winged hooks (Plate CVI, fig. 3 c) dilate very little above the base, and have a 

 marked forward curve throughout the greater part of their length, then a backward bend 

 below the neck, diminishing toward the latter. The main fang comes off at a consider- 

 able angle to the neck, is sharp, and the spike on the crown has a less angle with the 

 main fang than in P. ciliata. The wings are similar. 



The dorsal bristles become extremely slender posteriorly, but are of considerable 

 proportional length. Moreover the posterior region has bundles of bristles even more 

 slender than the foregoing in each foot (Mesnil states, dorsal). Each bundle contains 

 many, and they resemble linear crystals, being perfectly straight and slightly tapered 

 at each end. The function of these is unknown. Mesnil observes that they are entirely 

 caducous, and do not appear in front of the eighth bristled segment ; and at Guernsey 

 it was thought they were at the hook pads. The bacillary pouches seem only to contain 

 granules, larger than in Magelona. 



The branchiae commence on the eighth segment, attain their maximum size a few 



