20 TUBIFEX RIVULORUM. 



little in advance of the ventral setae ; and the internal orifice, after 

 traversing the system, opens into the segment preceding that in 

 which its convolutions and external orifice lie, where it presents a 

 crown of cilia. The normal function of these organs is believed 

 by d'Udekem and Claparede to be excretory, and not respiratory, 

 as has been supposed by others ; and this opinion seems to be 

 borne out by the fact that the ciliary current is always from 

 within^ outwards. In certain segments of the worm, however, the 

 segmental organs are specialised to provide efferent ducts, etc., 

 for the reproductive system. Where this is the case, they are 

 large and very apparent ; but in general they are rather difficult to 

 distinguish amid the contents of the perivisceral cavity. 



The alimentary canal consists of a mouth, pharynx, oeso- 

 phagus, and intestine. There is no muscular crop or gizzard, as 

 in the earth-worm. The mouth usually shows as a transverse line 

 on the lower surface of the cephalic segment. It opens into a 

 more or less globular pharynx, occupying two or three segments of 

 the body, and provided with a muscular coating. The pharynx is 

 capable of being projected from the mouth, and again withdrawn. 

 The oesophagus occupies the fourth and fifth segments, and is 

 succeeded by the intestine. As far as this point, the alimentary 

 canal is devoid of colour. The intestine is considerably larger than 

 the oesophagus, and extends to the termination of the body. It 

 has an inner mucous membrane, covered with vibratile epithelium, 

 the cilia being especially visible near the anus, and an outer 

 muscular coat, by which the peristaltic movements are effected. 

 This coat, however, requires the addition of acetic acid to render it 

 visible. It is covered throughout its length with a cellular invest- 

 ment (Figs. 8 and lo) — at least, such is the account generally given 

 of it ; but Lankester points out, what appears extremely probable, 

 that this cellular covering is but the internal parietes of the perivis- 

 ceral cavity. A glandular function is attributed to these cells, and 

 it is said that they secrete a fluid into the intestine analogous to 

 the bile of the higher animals. Claparede, however, doubts the 

 hepatic character attributed to them, pointing out that they cover 

 the dorsal vessel as well as the intestine ; and he thinks they may 

 pour their secretion into the perivisceral cavity. Lankester says : — 

 " The whole of the endothelium " {i.e., the whole of the wall of the 

 perivisceral cavity, including, as he views it, the glandular covering 

 in question) " sheds its cells into that cavity. The cells are filled 

 with brownish granules, giving a colour to the intestinal tract that 

 is wholly wanting to the pharynx and oesophagus." 



Before quitting our subject for the present, we may say that 

 d'Udekem describes the nervous system as consisting of two 

 closely united cords, bifurcating to form the oesophageal ring. 

 From the brain^ two pairs of nerves were thought to arise. The 



