ON ONCHNESOMA STBENSTEUPII. 221 



There is no closed vascular system such as exists in the 

 larger Sipunculids. The perivisceral 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). 



The Head. 



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, S. 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 



