222 ARTHtTR E. SHIPLEY. 



line the oesophagus. The cilia also cover the ventral lip. 

 The lobe is more or less solid (fig. 13), 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 oeso- 

 phagus, the retractor muscle is inserted ; it wraps round about 

 two thirds of the circumference of that tube (fig. 13). 



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 coelomic 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 coelom, 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 coelomic fluid ; this is spherical or nearly so, with 

 granular protoplasm and a well-defined nucleus (figs. 7 

 and 8). The coelomic fluid must be kept in very constant 

 motion, both by the ciliated cells of the peritoneal epithe- 



