Benham and Cameron. — Xephridia of Pcrii'oiliilus. 



193 



Perieodrilus (Plagiochaeta) ricardi. 



This worm was origmally stated to be micronephric (3, p. 287) ; but, 

 as has been pointed out in the introductory note, this is not the case, for, 

 though the appearance of the nephridia suggests separate tufts of tubules 

 arising independently from the body- wall, these tufts are all connected, 

 and constitute the looping of one long bundle of tubules. The worm, there- 

 fore, though the nephridium is diffuse, is meganephric ; but compared 

 with the very great size of the worm it is exceedingly small, is concentrated 

 near the nerve-cord, extends for about one-fiftli of the semicircumference 

 of the body-wall, and takes up but little space in the body-cavity. A 

 specimen measuring 13 mm. in diameter, when laid open, has its body- 

 wall 40mm. wide; the nephridium commences at a distance of 5mm. 

 from the nerve-cord, and the coils of the organ occupy only a length of 

 4 mm. ; the duct passes outwards to the pore in a straight line. 



It shows no trace of the distinct regions presented by such worms as 

 LuDibricus, Maoridrihis, &c. It consists of one long tubule of nearly uni- 

 form diameter, which is looped or folded vertically upon itself, the separate 

 folds being connected by horizontal tubules lying on the body-wall near the 

 base of the septum. The ventral ends of the upright folds and the hori- 

 zontal connections are bound together by blood-vessels and connective 

 tissue, and we may call this part of the nephridium the basal portion. 



The various folds have a diameter of about the same size as that of 

 the anterior or posterior folds of Maoridrihis, and the canals contained 

 therein are similar to those in the latter worm. 

 The specimens of P. ricardi which were examined 

 were all preserved, and the regions of the excre- 

 tory tube could not be clearly seen in all parts of 

 the nephridium, nor could they be traced indi- 

 vidally from funnel to pore. But the tubule is 

 differentiated into the narrow tube (o.o and «.?, 

 fig. 2), the ciliated middle tube (6), ("bridge" 

 of Maoridrilus), and the wide tube (c), as usual 

 in earthworms. The tubes follow the course of 

 each fold, bending back on themselves at the 

 apex or spur. There are several of these single 

 folds in each nephridium. and this fact, together 

 with the similarity in structure between the excretory canals of Perieodrilus 

 and Maoridrilus or other meganephric genera, leads one to think that the 

 nephridium of P. ricardi represents a transition state between a meganephric 

 and micronephric condition. The one large nephridium is still single, but 

 has become diffuse in form, and has lost the specialized regions of its 

 original form, though it has not yet broken up into separate tufts of micro- 

 nephric tubules. 



The outer and inner narrow tubes wind in and out of each other at the 

 apex of a fold in a manner which recalls the dorsal lobe of Maoridrilus : 

 the wide tube is not affected by these windings, but retains its direct course 

 round the fold. The narrow canals pass above and below it as they loop 

 round each other, and there is sometimes a loop round the wide canal. 

 Over a great part of the nephridium the inner narrow canal (fig. 2, a.i) Sends 

 out processes (a.hr) which loop round the other canals on the convex side 

 of the fold and return to the one from which they are given off. This 

 branching does not form a network such as occurs in the excretory tubides 



7— Trans. 



'^ CL.br. 



Fig. 2. — Perieodriltis ricardi. 

 Portion of a loop of a 

 nephridium mounted mi- 

 stained in glycerine. 



