208 ■ Memoirs of the Indian Museum. [Voiv. I, 



Circulatory system. — The general relations of the blood-vessels are the same as in 

 Nais. The ventral vessel appears to fork at a more posterior level, at about septum 

 6-7 ; and the transverse commissures in the œsophageal region are more numerous. 

 These occur regularly in all segments from the second to the seventh inclusive ; those 

 in the second segment are non-pulsatile, and are situated in the anterior part of 

 the segment, in front of the level of the setae of the segment ; in all the remaining 

 segments the commissures are pulsatile, and are placed posteriorly, lying on the 

 septum ; they increase somewhat in calibre as one passes backwards ; and the largest 

 and most obviously contractile are thus those of the seventh segment, which lie 

 against septum 7-8, just in front of the stomach. My description, therefore, does 

 not quite agree with that of Beddard [i, p. 291], who states that there are four 

 vascular arches, in segments v-viii; in another place [ih., p. 290] he admits 

 five. Michaelsen [3] also, in defining the species, places the vascular arches, six in 

 number, in segments iii-viii. As to the position of the most anterior commissure in 

 the second segment in the I/ahore specimens there can be no doubt; nor, I think, as to 

 the remainder being on the anterior, not the posterior, face of their respective septa. 



The nephridia commence regularly in the ninth segment, two segments behind 

 the corresponding position in Nais ; which may be correlated with the fact that the 

 genital organs in this genus also occur two segments further back than is usual in the 

 Naididae. Beddard [i] places the first nephridium in the tenth segment. They 

 possess a ciliated funnel, which projects through the septum into the next anterior 

 segment. 



The nervous system has the usual relations. The cerebral ganglion is deeply 

 indented in front and behind (plate xviii, fig. 31). It reaches behind to the level of 

 the dorsal setae of segment ii, in front to a level somewhat anterior to that of the 

 mouth. On one occasion when a specimen was viewed from the ventral surface 

 (fig. 32) the ventral nerve cord was seen to present a series of small " button-holes," 

 somewhat similar to those seen in Chœtogaster , in its anterior portion as far back 

 as the fifth segment. 



Sensory organs are apparently represented only by small hair-like projections 

 on the proboscis, and especially also at the hinder end of the body {cf. ant., External 

 characters). 



The reproductive organs will be described afterwards for the two species of the 

 genus. The same ectoparasite so often seen on Nais variabilis was also found here on 

 several occasions. 



P. longiseta appears to be a somewhat variable species, and Piguet [7] remarks 

 on the differences between his specimens, obtained in Switzerland, and those pre- 

 viously studied by Vejdovsky in Bohemia. The forms observed by me agree with 

 those of Vej dovsky in the gradual passage of the tentacle-like ' ' proboscis ' ' into the 

 pre-oral lobe at its base, whereas in Piguet's specimens the "proboscis" is sharply 

 marked off at its base ; and also in the comparatively moderate length of the dorsal 

 setae of the third segment. They agree, however, with Piguet's rather than with 

 Vej dovsky' s in the smaller number of dorsal setae per bimdle, in the facts that the 



