BORING AND SAND-FEEDING ORGANISMS. 341 



Apparently the same species often lives under beach sandstone masses, at Maduwari in 

 S. Mahlos being present in enormous numbers in this position. Spengelia was also obtained, 

 but, although a sand-feeder, was nowhere found in sufficient abundance for it to have any 

 appreciable effect on the sand. 



Next in importance come the species of the genus Sipimculus, which likewise possesses 

 a ciliated groove to the gut. Mr A. E. Shipley, who has had considerable experience of 

 tropical forms, in considering the Gephyi-ea (Cainb. Nat. Hist., vol. ii. p. 422) remarks, " The 

 food of Sipunculids seems to consist almost entirely of sand and their only nourishment must 

 be such small microscopic organisms or particles of animal or vegetable dehris as are to be 

 found mixed with the sand. The alimentary canal is as a rule quite full of sand, and yet 

 in spite of the tenuity of its walls they never seem to be ruptured. If the contents of 

 the digestive tube be washed out with a pipette, it will be found that it requires con- 

 siderable force to dislodge many of the sand-particles lying next the wall. These are more 

 or less imbedded in crypts or pockets of the wall, and, as the sand passes along the intestine, 

 they probably serve as more or less fixed hard points, against which the sharp edges of 

 the sand particles are worn off. Amongst the sand are usually to be found pieces of shell, 

 sometimes with a diameter equal to that of the alimentary canal; these are usually rounded 

 but their angles may have been removed by attrition before they entered the mouth of the 

 Sipunculid." 



The animals of the genus (Sipimculus) are extraordinarily abundant on all coral reefs 

 in certain positions, such as under dead corals or masses of rock. The quantity of sand 

 that passes through theii- guts must be considerable, but much of it seems to be retained 

 for a long period. It is too almost impossible by anaesthetisation to get the gut free from 

 sand. The castings are relatively both small and fine in texture. The quantity of sand 

 passing through the gut does not appear to be great, but what passes is much triturated. 

 Perhaps therefore the total action of these animals may be of more importance than that 

 of any other group. The species of our collection have already been reported on by 

 Mr Shipley, under each its distribution being noted. I may, however, remark that S. indicus 

 is found in large numbers on practically every sand flat in the Maldives. 



Of other sand-feeders Thalassema may be mentioned, of the contents of the gut of which 

 Shipley has given a list (p. 129). The genus is, however, rare on reefs. Polychaeta commonly 

 swallow as they burrow in the sand, but I have no reason to suppose that the sand forms 

 — although often very numerous — triturate the sand to any notable extent. This does not 

 quite agree with Mr Crossland's view (p. 337), but against this may be mentioned MTntosh's 

 observation that the deep-sea Challenger forms often had Foraminifera and Radiolaria in their 

 guts in an almost perfect state'. Much material, especially on the more muddy flats, must 

 nevertheless pass through their bodies, and cannot but be to some slight degree affected. 



Section VI. Beach Sandstone. 



Large, linear masses of a friable sandstone, extending sometimes for great distances along 

 the shores of the islands, are characteristic of coral atolls in whatever part of the world 

 they may be found. So far as I have seen in the Maldives and elsewhere, these masses 

 only occur in their tyijical condition (1) where there is a fringing reef or a sand flat of 

 greater or lesser breadth extending out from the shore, or where the beach is by other 



' "Annelida Polychaeta," Challenger Reports, p. ix. (1885). 



