DESCRIPTIONS OF GENERA AND SPECIES 277 



recorded in Aeolosoma. The nephridia, instead of being paired, are occasionally only 

 present to the number of one in each segment. In Nais heterochaeta, for example, this 

 appears to be the invariable condition ; moreover, in that species the nephridium 

 occupies the two segments following that in which the funnel lies ; hence, as Benham 

 (15, p. 385) remarks, there is really one nephridium in place of four. Here, however, 

 as in Phreatothrix, the external pore is invariably upon the segment immediately 

 following that which carries the funnel. This absence of one of the two nephridia of 

 a segment culminates in Uncinais littoralis. In this worm so careful an observer as 

 Bourne remarks (5, p. 351) that he 'was unable, even after repeated examination, 

 to discover any nephridia.' 



The nephridia in the Naidomorpha are not present in the anterior segments of 

 the body ; the segment in which they do commence varies according to the species, 

 and there is frequently an omission of a segment or segments in the series ; as, for 

 instance : — 



Bohemilla cofnata, viii, xi, xvi, xviii, xx, xxi. 



Pristina equiseta \ . „ 



Pristina breviseta ) 



Nais heterochaeta, vii, &c. (with some variation in individuals). 



Nais elinguis, viii, &c. 



That part of the nephridial tube which follows the funnel, but lies in the following 

 segment, is nearly always covered with a brown glandular investment, also found 

 in some of the Lumbriculidae and Tubificidae ; this appears to be absent in 

 N. lacustris. 



Among the Naidomorpha the vascular system shows various modifications. In 

 some it is more on a level with that oi Aeolosoma; in others it is distinctly like that 

 of the Tubificidae. In the latter group, which is represented by N. josinae, the dorsal 

 and ventral trunks are connected by a perienteric arch in all the segments of the 

 body; in the former group it is only in the anterior segments of the body that this 

 communication occurs directly. This being so, it is -possible to divide the Naido- 

 moi-pha into two groups, of which one is nearer to the higher Oligochaeta. It is 

 a matter of obvious interest to inquire whether the resemblance to the higher 

 Oligochaeta is secondary — whether, in fact, the species which show this particular 

 likeness are above or below the others in the scale. Fortunately, we are in 

 possession of evidence which enables this question to be decided with a great 

 amount of probability. Bouene has observed that in the buds of Uncinais 

 littoralis there are a series of vascular arches, one to each segment; but that on the 

 full-grown worm it is only the head-segments which are thus provided. 



