ON ONCHNESOMA STEENSTRUPII. 239 
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. 
Tue 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-ceils 
which cover in a fibrous portion on all sides except that 
nearest the cesophagus, 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 off three nerves; a median nerve to the 
median dorsal lobe, and one on each side, which pass round 
the esophagus 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 
1 On Phymosoma varians,” ‘Quart. Journ. Mier. Sci.,’ April, 1890. 
* On a New Species of Phymosoma, with a Synopsis of the Genus,” ‘ Quart. 
Journ, Micr. Sci.,’ March, 1891, 
