ON ONCHNBSOMA STEENSTRTJPII. 225 



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 difi'er 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. 



The Alimentary Canal. 

 The cilia which cover the dome-shaped dorsal process of the 

 bead and the lower lip are continued without break into the 



