Nephridia of Arcnicola 71 



Nkphridia. 



There are five pairs of nephridia in Arenicola pusilla, lovcni and 

 hranchialis, six pairs in A. marina, assimilis,^ cristata and glacialis, 

 and thirteen pairs in A. ccandata. 



Each nephridium may be divided into three regions — an anterior 

 funnel, a middle excretory portion, and a posterior vesicle or bladder. 

 The funnel, which is usually bright red in colour, owing to its rich 

 vascular supply, opens into the coelom by a slit-like aperture. The 

 larger dorsal lip of the funnel is fringed with ciliated vascular pro- 

 cesses, which increase in number and size as the worm grows. In 

 the caudate species these processes are flattened and spatulate or 

 triangular in shape, and their distal margin is usually subdivided into 

 several rounded lobes (PI. XIV, Figs. 48, 49, 50). In the ecaudate 

 species the processes are more cylindrical, and are more deeply divided 

 distally into two or three, or in large specimens two to six, finger- or 

 tliumb-shaped branches (PI. XV, Fig. 51). The ventral lip is not 

 fringed, and its margin is either entire and almost semicircular in shape, 

 as in A. marina, or it is deeply notched in the middle, as in the 

 ecaudate species. In A. assimilis the edge of the ventral lip of the 

 funnel is thrown into folds or frills (Fig. 50) ; a similar condition is 

 very occasionally met with in one or two other species, but in these the 

 " frilling " is much less marked than in A. assimilis. In A. cristata 

 the middle portion of the ventral lip is often thicker than the rest. 



Owing to the reduction in the number of septa in Arenicola the 

 typical relationship of nephridium to septum, as exhibited in many 

 Polychaeta and Oligochaeta, is seen in the adult of only three species, 

 and then only in regard to the first pair of nephridia. In A. marina, 

 assimilis ^ and glacialis the funnels of the first pair of nephridia are 

 situated on the anterior face of the third septum ; in the other 

 species the first nephridium is in the following somite. 



The excretory part of the nephridium is a thin-walled spacious 

 sac, usually dark brown, sometimes black, in colour, owing to the 

 presence of large numbers of brown excretory granules in the cells 

 lining the sac. This part of the organ tapers posteriorly and leads 

 into the contractile bladder, which opens to the exterior by a small 

 oval aperture situated near, and slightly liehind, the dorsal end of the 

 corresponding neuropodium. 



' South Afiican examples of A. assimiUs var. affinis have onl3' five pairs, 

 the nephridia of the fourth segment being wanting. 



- Except in South African examples of the var. affinii (see footnote '). 



