STRUCTURE OF UROCH2ETA AND DICHOGASTER. 265 
difference iu size is not very marked iu P. aspergillum, not 
nearly so much so as in P. armata. 
In my paper upon Perichseta aspergillum I have not 
figured the cuticular pores of the nephridia, and so I have 
thought it worth while to introduce into the present paper 
illustrations of their structures. Pig. 23 of Plate XXIV 
represents a portion of the cuticle of P. aspergillum, 
showing the cuticular ingrowths which surround the proximal 
region of the seta (a), and the very delicate cuticular tube ( b ) 
which lines the extremity of the duct of the nephridium. 
When these structures are viewed from above the aperture, 
whether of the seta or of the nephridium, they appear to be sur- 
rounded by a thickened layer of the cuticular membrane. This 
is, I believe, only an optical effect due to the inturned edges of 
the cuticle. It seems, however, to define very plainly the orifice. 
The very great size of the seta orifice, as compared with that 
of the nephridiopore, will be evident from an examination of 
the figure cited. The cuticular pore of the nephridium is 
further remarkable for the fact that its edges are usually 
much crinkled, which is probably due to the contraction of 
the epidermic cells by the preservative reagent. The cuticular 
pores which lead into the seta sacs never show these crinkled 
edges, probably for the reason that they remain distended by 
the seta. 
Having ascertained that these cuticular pores belong to the 
nephridial system, I have examined the cuticle of another 
species of Perichseta of which I possess examples not suf- 
ficiently well preserved to show the modifications of the 
epidermal cells round the nephridiopore. 1 find that they 
are present in Perichseta Iloulleti, and I consider myself 
therefore at liberty to infer that iu this species (and, indeed, 
probably in all Perichseta in which the nephridia have a 
“tufted” character) the structure of the nephridial system is 
much the same as that of P. aspergillum. 
In Acanthodrilus and Dichogaster the external orifices 
of the nephridial system are larger than those of Perichseta 
and (judging from Spencer’s description) of Mcgascolides; 
