ON ONCHNESOMA STEENSTRUPII. 227 



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 debris ; 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 role 

 in bringing this about. 



These processes must be mainly effected by Holothurians, 

 Echinids, Polychsetes, 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 



• "On the Occurrence of Sulphur in Marine Muds and Nodules, and its 

 Bearing on their Mode of Pormation," J. Y. Buchanan, ' Proc. of the Rojal 

 Soc. of Edinburgh,' Dec, 1890. 



