ON ONOHNESOMA STEENSTRUPII. 219 



the body. The skin of the introvert, when extended, is traES- 

 parent, so that the oesophagus and nervous system may be seen 

 through it. 



There is a marked thickening of the skin where the introvert 

 joins the body; the anus is situated a little anterior to this. 

 The external opening of the kidney is a little behind^ just to 

 the side of the ventral nerve-cord. 



The Structure of the Skin. 



The layers which compose the skin of Onchnesoma have 

 been described by the Norwegian writers ; there are, however, 

 one or two details which may be added to their account. As 

 in the skin of other Sipunculids, we meet with six layers. 

 The state of preservation of my specimens did not allow the 

 epidermal cells to be made out. But this outermost layer of 

 cells probably forms part of the deeply stained external layer 

 seen in fig. 6. 



In section this layer appears ridged, the ridges corresponding 

 with the wrinkles on the surface of the animal. It is obscured 

 by a number of granules, which stain very deeply with hsema- 

 toxylin ; these granules are apparently produced by certain 

 structures which correspond with the epidermal glands of 

 other forms. Between the darkly stained external layer 

 and the circular muscles is a thick gelatinous connective- 

 tissue layer, or cutis, in which hardly any trace of cells 

 could be detected. At the base of this the skin glands are 

 situated. 



The state of the preservation of my material did not allow 

 me to see this point very clearly, but I have no doubt that the 

 epidermal glands are composed of specialised epidermal cells, 

 Each gland is of a spherical shape ; from the outer edge of this a 

 duct with sharply defined outline leads through the cutis to 

 the surface of the body. Within the glands lie numerous darkly 

 stained granules, similar to those which cover the outside of 

 the body ; and there is little doubt that the latter have their 

 origin in these structures, and pass out with the mucus which 



