THE ENTEROPNEUSTA. 637 
its dermal pits and median gonads, it exhibits structural differences sufficient to separate 
it at once from Sp. porosa and Sp. alba, the only two species of this genus hitherto described. 
Willeyia bisulcata, nov. gen., n. sp. (Pl. XXXVIII. figs. 14, 23; Pl. XXXIX. fig. 29; 
Pl. XLI. figs. 9, 10; Pl. XLII. figs. 12—16). 
Locauity, ETc. This species was fairly abundant at Zanzibar, whence portions of two 
specimens were brought back by Mr Crossland. They were obtained by digging in mud 
on which Zostera was growing at low-water mark. 
EXTERNAL FEATURES. To judge from the fragments the length of this species must in 
the entire state be considerable. The proboscis is about double the length of the collar 
and is marked both dorsally and ventrally by a deep median groove (Pl. XXXVIII. fig. 14). 
Its width is somewhat greater than its dorso-ventral depth. The collar is long and is 
somewhat broader behind than in front. Near its posterior end there is a well-marked 
circular groove. In one of the two specimens available, which was somewhat larger than 
the other, the length of the proboscis was 14 mm., that of the collar 7mm. The average 
breadth of both was about 4mm. The branchial region and trunk have each a breadth 
somewhat less. In the specimen above alluded to some 32 mm. of the branchial region 
were still attached to the collar. No external liver saccules were visible on any of the 
fragments. Probably they are not present. No portion of the caudal region was preserved. 
With the exception of the proboscis, which is somewhat dorso-ventrally flattened, the whole 
body of the worm is more or less cylindrical. When living the animal was of a uniform 
cream colour. 
Proboscis. The epithelium is high though almost destitute of glands. It shews a very 
distinct layer of elongated nuclei about its middle and numerous small rounded nuclei in its 
deeper portion. The layer of nerve fibrils is very distinct and well developed throughout 
this region and rests on a fine but clearly marked basement-membrane. The circular muscle 
layer is strong, being nearly half as thick again as the nervous layer. The longitudinal 
muscles are well-developed, filling the greater portion of the proboscis (Pl. XLII. fig. 12), 
The nervous layer is thickened in the grooves. 
The proboscis coelom consists of a right and a left cavity which are entirely separate 
throughout. They are lined by a well-marked epithelium which is considerably thickened 
near the middle line (Pl. XLII. fig. 12 and Pl. XXXIX. fig. 29). The two divisions of the 
coelom are surrounded by a common layer of circular muscles (Pl. XLII. fig. 12). The left 
division of the proboscis coelom communicates with the exterior by a large pore (Pl. XLI. 
fig. 9). The right division ends blindly. No dorsal septum is present but there is a well- 
marked ventral septum, at the upper edge of which runs the ventral recurrent vessel of 
the proboscis (Pl. XLII. figs. 11 and 12, and Pl. XXXIX. fig. 29, wrv.). This septum 
springs from almost the tip of the proboscis and does not become complete until shortly 
behind where the pericardium arises. Here the ventral vessel becomes connected with the 
vessels of the glomerulus. On either side of the middle line are found dorso-ventral 
muscles, the dorsal ones being more marked than the ventral. Both converge towards the 
vermiform process of the stomochord where this structure is found (Pl. XXXIX. fig. 29). 
The stomochord exhibits a well-marked lumen throughout, both in its central portion 
and in the lateral diverticula. Anteriorly it is produced forwards as a vermiform process 
which is about 2 of the length of the rest of the stomochord (Pl. XLII. fig. 10). 
G. IL 82 
