ON ONOHNESOMA STEENSTRUPII. 223 



lium, and by the alternate protrusion and retraction of the 

 introvert. 



In the two species of Phymosoma which I have described ^ 

 there is a very extensile fold of skin or collar which surrounds 

 the base of the head, and which, when the introvert is re- 

 tracted, usually completely encloses the head. The function 

 of this collar is perhaps to shield and protect the delicate 

 ciliated tentacles and lips from contact with the indurated 

 surface of the introvert, provided as it often is with horny 

 hooks. No such collar is found in Onchnesoma. 



The Nervous System. 



The brain is an elongated mass situated dorsal to the mouth, 

 at the base of the median dorsal ciliated lobe (fig. 11). It 

 shows no trace of being bilobed. Like that of Phymosoma, 

 the brain of Onchnesoma consists of a cap of ganglion-cells 

 which cover in a fibrous portion on all sides except that 

 nearest the oesophagus, the ventral (fig. 12). There are no 

 giant ganglion-cells to be seen. The nerve-cells are all of one 

 size, with nuclei which stain deeply. On the dorsal surface 

 the brain is continuous with the epidermis; but in this region, 

 just at the base of the median dorsal process, the epidermal 

 cells are not in any way modified. The pigment which accu- 

 mulates in similarly placed cells in other Sipunculids is 

 absent. There are also no eyes. 



The brain gives ofi" three nerves ; a median nerve to the 

 median dorsal lobe, and one on each side, which pass round 

 the cEsophagus and fuse together to form the ventral nerve- 

 cord (figs. 9, 10, and 11). The median nerve is doubtless the 

 equivalent of the pair of nerves which supply the pigmented 

 pre-oral lobe in Phymosoma. The median lobe is probably 

 sensory and tactile, and is therefore supplied with a stout 

 nerve. The second pair of nerves in Phymosoma, which supply 



* " On Phymosoma varians," 'Quart. Journ. Micr. Sci.,' April, 1890- 

 " On a New Species of Phymosoma, with a Synopsis of the Genus," 'Quart. 

 Journ. Micr. Sci.,' March, 1891. 



