ON ONCHNESOMA STEENSTRUPII. 237 
There is no closed vascular system such as exists in the 
larger Sipunculids. The periviscerai fluid which bathes the 
internal organs is crowded with nucleated corpuscles and gene- 
rative cells. 
The single kidney varies in position; in some of my speci- 
mens it was situated to the left of the ventral nerve-cord, in 
others to the right. Both its internal and external openings 
are too small to be made out except by section. The ventral 
nerve-cord may be seen as a very fine strand running just 
inside the skin (fig. 8). 
Tue Heap. 
The head of Onchnesoma is of a remarkably simplified 
nature compared with that of the larger Gephyrea, but 
whether the simplification is primitive or the result of dege- 
neration is not an easy matter to decide. The hooks which 
are so common in the group, arranged in rings round the 
proboscis, are entirely absent in this genus, This is a point 
of some interest taken in connection with the absence of 
several other structures which are usually met with in the 
group, but too much stress must not be laid on it, as with one 
exception, 8S. australis, the whole genus Sipunculus is 
devoid of these structures, and in other genera several species 
are without hooks ; they are also apt to drop off as the animal 
grows old. 
A more important feature is the entire absence of any ten- 
tacles. There is no trace whatever of the lophophoral ring of 
tentacles such as occurs in Phymosoma, and the crumpled 
pigmented tissue which occupied the hollow of the horseshoe 
is also entirely absent. The place of these structures, in the 
dorsal side of the mouth, is occupied by a slight elevation or 
blunt process which contains the brain. This process has a 
slight resemblance to a Doge’s cap, but it is really nothing 
more than an extension of the body-wall on the dorsal side of 
the thickened lip which surrounds the mouth. The skin 
covering this process is not pigmented, but the whole of it is 
uniformly ciliated, the cilia being continuous with those which 
