BORING AND SAND-FEEDING ORGANISMS. 339 



Holothuria maculata, was most conspicuous everywhere on the sand flats, attaining a length of 

 3 feet when extended. It makes burrows, up to 6 feet in depth, sometimes bent with two or 

 more openings. At Minikoi, where the sand flat of the lagoon near the light-house was at low 

 spring tides uncovered for some distance, the drying up appeared to stimulate these Holothurians 

 to actively extrude their sand, and the whole flat every yard or so was studded with little masses 

 of their pellets, which at once on the tide rising broke down into sand-heaps. 20 out of 52 or 

 these castings from an especially rich area were scraped up and measured, giving an average 

 of 37 c.c. for each animal. This, if spread out evenly, would cover the whole area to a depth of 

 •11 mm., or a layer of sand of over 40 mm. in thickness might be supposed in this area 

 annually to pass through the bodies of this one species. In addition in every sieve full of 

 sand from this same area large numbers of pink Synaptids, about 2 inches long, were obtained, 

 which, if less conspicuous, probably passed still more sand through their bodies than the larger 

 form just mentioned. 



Holothurians are especially adapted to sand-feeding, in that they possess a conspicuous, 

 ciliated groove to the gut. Besides extruding pellets many species also, or perhaps always, 

 pass the sand through their bodies in a stream. The surface forms, mentioned above, feed 

 during the night, remaining during the day more or less dormant. Of C3 specimens of 

 Stichopus chloronotus, obtained at dawn between the large island at Minikoi and Wiringili, 

 59 had sand in the mouth part of the gut. This was absolutely the same as that covering 

 the bottom, mostly clean sand with a little fine washings. The output of these forms seemed 

 almost continuous during the day, and consisted of fine sand only. Of 31 specimens obtained 

 about mid-day all had the fore part of the gut free and the mid part choked with coarse 

 sand. A few, kept in a barrel and not allowed to feed, extruded some of their sand in pellets, 

 but most retained it for 3 or 4 days, only getting rid of the finest particles. In ten 

 specimens, kept in a sunken barrel and allowed to feed on fine sifted sand only, the mid 

 part of the gut was found after 5 days to be still filled with coarser particles than any, which 

 the animals had been given. 



A Holothurian dredged from 43 fathoms, E.N.E. of Havaru-Tinadu, Suvadiva, was singularly 

 interesting. It came fi-om an area of the finest mud, which extended for 1^ miles in all 

 directions from the spot where it was dredged. The mid-gut was, nevertheless, full of relatively 

 large fragments of coral, Halimeda leaves, etc., the coarseness of the contents approaching such 

 sand as ordinarily covers the bottom at 20 fathoms under a relatively strong current. 



The sand of the mid-gut of all reef Holothurians, that I have examined, is always seen to 

 be clean and the particles more or less rounded. In addition there is little loss on washing, 

 while the sand in the first 3 inches of the gut commonly loses 2 — 8 per cent, of its weight in 

 this process. Indeed, there is no doubt but that the coarser particles are detained in the gut 

 for a considerable time — and probably worn down — while the sand and mud is by some means 

 — presumably the ciliated groove — passed onwards. The extrusion of the whole gut and its 

 subsequent regeneration is a process which commonly occurs in nature. It, not improbably, is 

 for the jjurpose of getting rid of the very coarse sand, the exertions of the animal not being 

 able to force it out in the form of pellets. 



Of course different kinds of Holothurians may behave in diverse ways, but the total effect 

 even of the two surface forms mentioned above must be enormous. The weight of sand, 

 when dried, from 4 average specimens of Holothuria atra, taken in the early morning, was 

 297"3 grams. Of this 44"3 grams were extruded as fine particles into the bucket in 5 hours. 



