ON ONOHNESOMA STEENSTRUPII. 231 



velopment whilst floating in the coelomic fluid. They leave 

 the testis in the condition of the mother-cells of the sperma- 

 tozoa ; these segment, and the sperm morulse result. The 

 spermatozoa keep together in sperm morulse till they have 

 passed through the nephridium and out of the body. 



Conclusions. 



Onchnesoma is the smallest Sipunculid with which we are 

 acquainted, and its anatomy is to a considerable extent more 

 simple than that of other members of the group. 



The head is much simplified j the lip which surrounds the 

 mouth bears no tentacles, but is produced dorsally into a blunt 

 process covered with cilia. The simplicity of structure is 

 shown by the absence of any tentacles, hooks, collar, pig- 

 mented skin, and eyes ; there is also no vascular system, 

 no spindle muscle, and no giant-cells are found in the brain. 

 The retractor muscle is single and arises from the extreme 

 posterior end of the body, and is, therefore, symmetrical j the 

 nephridium is also single, and may lie to the right or left of 

 the body. The brain is not bilobed. 



Until we know something of the development of Onchne- 

 soma it would be hazardous to express an opinion as to whether 

 the absence of the above-mentioned organs is due to degenera- 

 tion, or whether they are primitive. On the one hand, the 

 small size of the animal and the presence of one nephridium, 

 which occurs on either side of the median line, points to 

 degeneration ; whilst, on the other, the structure of the head 

 indicates a primitive condition, which might admit of modifi- 

 cation in various directions. 



The absence of any closed vascular system, correlated with 

 the absence of tentacles, may throw some light upon the vexed 

 question of the seat of the respiratory processes in Sipunculids. 

 Since there are no tentacles, there is one Sipunculid at least 

 which does not breathe by them ; and although I think, when 

 they are present, some respiration may be carried on by them 

 and the closed vascular system, especially in reference to the 



