xv SIPUNCULOIDEA FOOD 423 



The enormous amount of sand and mud which passes through 

 the bodies of the Sipunculids shows that they must take a con- 

 siderable part in modifying the mineral substances which form 

 the bottom of the sea. Just as earthworms, as shown by Darwin, 

 play a considerable role in the formation of soil, so must these 

 animals, in conjunction with Echinids and Holothurians, effect 

 considerable modifications in the sand and mud which pass 

 through their bodies. Mr. J. Y. Buchanan l is " led to believe 

 that the principal agent in the comminution of the mineral 

 matter found at the bottom of both deep and shallow seas and 

 oceans, is the ground fauna of the sea, which depends for its 

 subsistence on the organic matter which it can extract from the 

 mud." The minerals at the bottom of the sea are exposed to a 

 reducing process in passing through the bodies of the animals 

 which eat them, and subsequently to an oxidising process due 

 to the oxygen dissolved in the sea-water acting on the minerals 

 extruded from the animals' bodies. 



The rate at which the sand passes through the body of 

 Sipunculus is unfortunately unknown, but that at any one 

 moment a considerable quantity is contained in the intestine is 

 shown by the fact that the average weight of five specimens of 

 S. nndus from Naples, taken at random, was 19'08 grms., whilst 

 the average weight of sand washed out of their alimentary canal 

 was 10'03 grms. The sand contained in five other specimens 

 of the same species measured respectively 6 c.c,, 7 c.c., 6'5 c.c., 

 7*5 c.c., and 7'5 c.c., giving an average of 6'9 c.c. for each 

 individual. 



Onchnesoma and Tylosoma have only one retractor muscle ; 

 Aspidosiphon and Phascolion have, as a rule, two ; Phymosoma 

 and Sipunculus have four, and perhaps this is the more usual 

 number. 



Phascolion, Tylosoma, and Oncknesoma have but one " brown 

 tube " ; in Phascolion this is the right, in Onchnesoma it is 

 sometimes the right and sometimes the left that persists. Most 

 other genera retain two, but there are many exceptions; for 

 instance, Phascolosoma squamatum has but one, and so has 

 Aspidosiphon tortus, and in both cases it is that of the left 

 side. No Sipunculid has more than two. It has been pointed 

 out by Selenka that those species which have but one brown 

 1 Proc. Roy. Soc. Edin. xviii. 1892, p. 17. 



