ON ONCHNESOMA STEENSTRUPII. 243 
The whole alimentary canal is attached to the body-wall by 
a few fibrous strands, but there appears to be no spindle muscle 
running up the axis of the spirally coiled intestine. 
The food of Onchnesoma, judging by the contents of the 
intestine, consists of vegetable débris ; mixed with this is a con- 
siderable amount of sand and a number of spicules, whose 
precise nature I was not able to make out. 
The enormous amount of sand and mud which passes through 
the body of the Sipunculids shows that these animals must 
take a considerable share in the reducing action to which the 
mineral substances at the bottom of the sea are subjected. 
Mr. J. Y. Buchanan has recently published an interesting 
paper ‘“ On the Occurrence of Sulphur in Marine Muds,”? in 
which he has drawn attention to the fact that most silicates 
are to some extent soluble when pulverised under water, and 
the sand is to some extent crusted in passing through 
the body of most mud-eating animals, and this solubility 
is increased by the sulphates in the sea water which passes 
through the intestine of the animals. The sulphates are 
reduced by the organic products of the body to sulphides, and 
these unite with the iron or manganese of the silicates, and 
leave the body as sulphides of iron or manganese. These sul- 
phides are then oxidised by the oxygen which exists in sea 
water, and form the red clays and chocolate muds which cover 
a considerable extent of the bottom of the sea. Thus the con- 
stitution of the mud at the bottom of the sea is to a very large 
extent artificial, and the Sipunculids play a considerable réle 
in bringing this about. 
These processes must be mainly effected by Holothurians, 
Echinids, Polychetes, and Sipunculids ; and to arrive at some 
sort of an estimate of the amount of sand taken into the body 
of the latter animals, I recently weighed five specimens, 
chosen at random, of S. nudus from Naples, and then weighed 
the sand in their intestines. The average weight of their body 
1 “On the Occurrence of Sulphur in Marine Muds and Nodules, and its 
Bearing on their Mode of Formation,” J. Y. Buchanan, ‘Proc. of the Royal 
Soc. of Edinburgh,’ Dec., 1890. 
