OCR Text |
Show 1904.] MARINE FAUNA OF ZANZIBAR. 329 The feet of the anterior part of the body project considerably, those of the posterior very little. The former appear to be thicker distally than at their bases, owing to the breadth and thickness of the lips of the seta-sac and to the swelling of the ventral cirri to an egg-shape, the broad end being the distal. The posterior feet are as usual pointed at their tips and have small, scarcely visible ventral cirri. In this way is caused the more striking of the differences between the anterior and posterior parts of the body, the change taking place quite rapidly at the point mentioned. at which also a two-hooked acicular seta first appears. In the anterior feet the dorsal cirrus is long, projecting beyond the end of the seta-sac; posteriorly it is very small (compare the figures of the feet in text-fig. 66). Anterior and hinder feet of Nicidion gracilis. X 00. The seta? are numerous and project well anteriorly when not. as is frequently the case, broken off level with the end of the seta-sac. This is the case almost always posteriorly, where usually only a fewT comb-seta? are found projecting. All are of the usual form and their details are given in fig. 11, P L X X I I. There is a single pair of short and slender anal cirri. This species is very closely related to N. brevis Ehlers, from which it is distinguished at once, however, by the slenderness and anterior compression of its body, the shortness of its tentacles. and form of its teeth. The resemblances between the two species are certainly more striking than the differences, comprising the smallness of the tentacular cirri, broad features of the jaw-apparatus (including apparently the remarkable delicacy of the lower jaws), structure of the feet and setae, and the characteristic differences between those of the anterior and posterior portions of the body. |