234 PCECILOCH^TUS SERPENS. 



least half the length of the body, slightly tapering at the distal extremity, and being 

 horseshoe-shaped (from a groove) in transverse section. A single blood-vessel occurs in 

 each, and probably, as in Magelona, they subserve respiration. From the posterior dorsal 

 region three long brownish tentacles, viz. a median and two lateral, arise from a single 

 broad base, and these are considered by Allen to be the nuchal organ. 



The short proboscis with thick walls is generally retracted in preserved specimens, 

 and behind the intestine is comparatively straight, but especially from segment sixteen 

 backward it is dilated in every segment, and the pouches are constantly contracting, so 

 that the circulation is thus aided. A deep ciliated groove occurs along the mid-ventral 

 line of the intestine throughout. 



Parasitic nematodes frequently occur encysted in the body-cavity. 



In the anterior region (segments one to eleven) is a large muscular blood-vessel, the 

 contractions of which are from behind forward, and from the third to the eleventh seg- 

 ment a lateral branch runs outward and downward on the posterior septum, then sends 

 a twig to the foot, and, branching, one of the trunks divides into a number of blind finger- 

 shaped processes in the segment behind. Anteriorly the dorsal vessel bifurcates, sending 

 a large vessel to each palp which would appear to have a respiratory function. In the 

 middle region of the body (segments twelve to fifteen) the dorsal trunk is much enlarged, 

 with lateral pouches on each side, and the expansion and contraction of these is a con- 

 spicuous feature, valves occurring between each successive pair of pouches. In the 

 posterior region the dorsal vessel can no longer be distinguished, but is merged in a sinus 

 which surrounds the intestine, as in certain Qligochasts. 



The body is long, about 55 mm. and having 110 segments, slender and cylindrical 

 anteriorly, but diminishing a little posteriorly, as well as being flattened, a concavity, 

 indeed, appearing dorsally ; moreover, the dorsal cirri are only half the size of the ventral 

 (which are nearly normal in size). The pygidium (Plate 01, fig. 3 a) has the anus dorsal 

 in position in the midst of five lobes, and with a pair of long dorsal and a pair of short 

 ventral cirri. The mouth is ventral in the midst of ridged lips. The proboscis is almost 

 spherical, and covers the median tentacle on the base of its anterior wall. 



The colour of the anterior region is bright scarlet or deep purple, according to the 

 degree of aeration of the blood, the feet and cirri being colourless. The posterior region 

 is dark green or white, the former colour being due to the intestinal cells, the latter to the 

 male elements. 



The first segment (peristomium) is largely developed, and bears a foot composed of 

 fused dorsal and ventral divisions, the cirrus of the latter being large, flask-shaped, and 

 directed forward, whilst that of the former is rudimentary. The bristles in each are simple, 

 long, smooth, and curved inward, so that they often cross. Slight differences occur in 

 these in regard to length and slenderness (Plate 0V, fig. 18, and Plate CXI, fig. 11), 

 and almost all are finely striated longitudinally throughout the greater part of their 

 length, only the curved tip having a tendency to a finely granular centre. Small papillaa 

 cover the feet and cirri, and at their ends are the openings of mucous glands. Between 

 the dorsal and ventral cirri are small, projecting pear-shaped sense-lobes, with hairs pro- 

 jecting from a cup-like depression at the extremity. 



In the second segment the dorsal cirrus is well developed, and the intermediate sense- 



