THE ANATOMY. NEPHRIDIA 43 



for some distance in its thickness before opening on to the exterior ; in this case 

 they generally run between the two muscular coats. 



In Perichaeta the nephridial tubules are of finer calibre than: in Octochaetus j 

 and they are furnished with funnels, which are especially obvious in the anterior 

 region of the body ; in this region, apparently in all true Perichaetae — it has at any 

 rate been commented upon by many observers — the nephridial mass is much thicker 

 than posteriorly ; the septa are here covered with a thick almost furry coating of 

 tubes, which in sections are seen to leave but little of the available coelom free ; 

 further back the nephridia are by no means so obvious, a fact which has led 

 to their being described as absent by myself (in Megascolex coeruleus for- 

 instance) ; in poorly preserved earthworms they would not been seen by anyone 

 who was not already informed of their existence and therefore unprepared for their 

 excessive minuteness. I have already mentioned that to Benham belongs the credit 

 of having first discovered the existence of numerous minute nephridia in a species 

 of Perichaeta; from his account it would appear that in this species the several 

 nephridia are quite distinct from each other ; this does not seem to be the case 

 in all the species of this genus ; in Perichaeta hermudensis, for example, it seemed 

 to be undoubtedly the fact that there was a connexion between all the nephridial 

 tubes of a segment, and that in addition to this there was a connexion between the 

 nephridial plexuses of following segments ; the tubes were followed through the septa ; 

 they appeared to pass into the plexus of the segments in front and behind. The 

 exact arrangement of the nephridial tubes is very difficult to follow on account 

 of their smallness and complicated course, but at any rate Spencer's investigations 

 into the anatomy of the excretory system of Megascolides led him also to the 

 conclusion that there was a connexion from segment to segment. That there are 

 numerous external pores is a matter capable of being easily proved ; if a bit of the 

 cuticle be stripped off, the pores can be easily seen, particularly in specimens which 

 have been preserved in corrosive sublimate ; the pores, although small, are far from 

 invisible; it is a matter of interest that there seems to be no regularity in their 

 arrangement; there is no trace of any metameric disposition. The segmentation 

 so clearly visible in most of the organs of the worm's body has here been lost. 



The minute structure of the nephridia in those worms which have an excretory 

 system of the diffuse type has been chiefly studied by Spencer (1) and myself (45, 

 47). Some details about the funnel are to be found in Benham's paper already 

 referred to. As Benham has pointed out, the nephridia of Perichaeta evidently consist 

 of the three regions described by himself in Lumhricus ; in Octochaetus only two kinds 

 of tubes are to be found. 



G 2 



