264 Memoirs of the Indian Museum. [Vol.. I, 



" hook-setse" are one or two in number in each bundle. The bundles of the dorsal 

 setse begin in the sixth segment, and are perhaps most commonly composed merely 

 of one hair- and one hook-seta. 



Septa are well marked; the corpuscles of the body-cavity resemble the white 

 corpuscles of Nais variabilis ; brown corpuscles were not seen. 



The remaining systems require only brief mention. The pharynx was seen to be 

 ciliated; there was no differentiation of a stomach, the whole of the alimentary 

 canal behind the pharynx having the same character throughout ; its walls contained 

 brown oil-like globules, as in the previous form. The usual ''antiperistaltic" 

 movements were observed. The œsophageal or pharyngeal commissures were plexi- 

 form and irregular. The first nephridium was in the seventh segment. In other 

 points this form appeared to correspond entirely with the Nais previously described. 

 The points of distinction consist in the absence of eyes, the forked sickle-shaped 

 dorsal setse, and the pharyngeal plexus. The sickle-shaped setse appear to be 

 identical in form with those of N. heterochceta, Benham, which species, however, posses- 

 ses eyes. But the form which comes closest to the present one is N. paraguayensis , 

 Mchlsn. (Michaelsen [4]), recently described from Paraguay; there appears to be 

 a slight difference in the hooked dorsal setse, no sickle-shaped curve being figured 

 or described by Michaelsen for his form; the forking, however, is of the same nature, 

 and there would hardly seem to be sufficient reason for separating the two, though 

 my specimens appear to be considerably larger than those from Paraguay. 



Pristina longiseta, Ehrbg. (PI. xvii, fig. 25.) 



External characters. — Specimens have an average length of 3 — 4 mm. The animal 

 is whitish in colour, and fairly transparent. The prostomium is prolonged into a 

 proboscis-like projection, which, during the forward progression of the animal, is 

 frequently bent backwards, at least when examined in the usual way under a 

 cover-slip ; it is in length about equal to the diameter of the body {v. plates xvii, xviii, 

 figs. 25, 29). The number of segments in the single animal appears to vary between 

 about 20 and 30 (see also below, Asexual reproduction). Backward progression is as 

 free and almost as often resorted to as forward progression, and indeed some specimens 

 have appeared to move backwards for choice ; in this connection it may be noted that 

 the posterior end of the body is specially well furnished with large sensory '' hairs." 



Asexual reproduction. — The greater number of specimens examined exhibited 

 some phase of the process of fission. In one case, after the twelfth segment intercala- 

 tion of new segments had taken place, evidenced by small newly-developed setal 

 bundles ; of these there were ten pairs, four belonging to the anterior and six to the 

 posterior animal. In a second case, intercalation had again taken place behind the 

 twelfth segment, and five newly -formed setal bundles belonged to the anterior animal, 

 six, as before, to the posterior; 15 segments had thus been established in the 

 anterior animal, and others were probably in process of formation at the posterior 

 end; while in the posterior animal 21 were established, and others were apparently 

 being produced posteriorly. In a third case the setse were apparently well grown as 



