ON ONCHNESOMA STEENSTRUPII. 247 
velopment whilst floating in the ccelomic fluid. They leave 
the testis in the condition of the mother-cells of the sperma- 
tozoa; these segment, and the sperm morule result. The 
spermatozoa keep together in sperm morule 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 ; 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 ; 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 
