ON ONCHNESOMA STEENSTRUPII. 241 
The walls of the nephridium are lined throughout, with the 
exception of the small area between the external and internal 
opening, with glandular cells of a considerable size; with the 
exception of the ova they are the largest cells in the body 
(fig. 4). The lumen of the kidney in Onchnesoma is not split 
up into a series of crypts communicating with a central cavity, 
as was the case in Phymosoma; and the cells do not get rid of 
the product of their secretion by breaking off a bubble from 
their free end. Each of the large columnar cells has a very 
definite outline ; their protoplasm is very clear and does not 
stain well, but scattered through it are a great number of 
granules which stain deeply. These concretions differ in size ; 
they are always spherical, and the larger ones have a double 
contour. These latter are often found in the lumen of the 
nephridium, having doubtless passed out of the glandular cells, 
and being on their way out of the body. 
I have never seen ova or spermatozoa in the lumen of the 
kidney, though I have no doubt that they leave the body 
through this channel. 
The muscular layer is not so well developed in the nephridium 
of Onchnesoma as in that of some other Sipunculids, and the 
size of the kidney was more constant. Covering the outside of 
the organ is a layer of peritoneal epithelium. 
With regard to the number of nephridia, two is undoubtedly 
the normal number in the Sipunculids; the genera Phasco- 
lion, Tylosoma, and Onchnesoma being singular in having but 
one. There are, however, exceptions to this rule: thus Phas- 
colosoma squamatum has but one, and Aspidosiphon 
tortus also retains but one; and in both these cases it is the 
left that persists. Some species of Phascolion, on the other 
hand, retain the kidney of the right side only ; and in Onchne- 
soma sometimes the left and sometimes the right persists, but 
never both together. 
Tue ALIMENTARY CaNAL. 
The cilia which cover the dome-shaped dorsal process of the 
head and the lower lip are continued without break into the 
