238 ARTHUR E. SHIPLEY. 
line the cesophagus. The cilia also cover the ventral lip. 
The lobe is more or less solid (fig. 12), and contains the brain, 
the rest of the space being filled up with connective tissue. The 
brain gives off a median nerve (figs. 10 and 11), which passes 
into the lobe, and is distributed, I believe, to the epidermal 
cells, so that doubtless the lobe has a tactile and sensory 
function. 
Just beneath the brain, on the dorsal surface of the ceso- 
phagus, the retractor muscle is inserted ; it wraps round about 
two thirds of the circumference of that tube (fig. 12). 
Corresponding with the absence of the tentacular crown 
there is a total absence of any vascular system, a peculiarity 
which Onchnesoma shares with Petalostoma and Tylosoma. 
There can be no doubt that in those forms which possess ten- 
tacles they have both a tactile and sensory function, and that 
they serve, by the currents their cilia give rise to, to bring 
food to the mouth. It is also believed that they have a respi- 
ratory function; and though this is probably the case, it must 
not be overlooked that the above-mentioned genera manage 
to respire without tentacles. Where the exchange of gases 
takes place is not so easy to state. The skin of Onchnesoma is 
relatively to the size of the animal at least as thick as that of 
the larger Sipunculids, and is covered by a thick cuticle. It 
has occurred to me that the celomic fluid may possibly obtain 
the oxygen it requires from the water which passes through 
the intestine of the animal. The coiled nature of this tube 
exposes a very considerable area to the fluid in the ccelom, and 
the extreme thinness and delicacy of its walls would favour a 
ready exchange of gas. If such a function were exercised by 
the alimentary canal, it would possibly explain the thinness of 
the digestive walls, which in other respects seems ill adapted 
to a diet of sand. 
In Onchnesoma there is only one kind of corpuscle in 
the cclomic fluid; this is spherical or nearly so, with 
granular protoplasm and a well-defined nucleus (figs. 7 
and 8). The celomic fluid must be kept in very constant 
motion, both by the ciliated cells of the peritoneal epithe- 
