BORING AND SAND-FEEDING ORGANISMS. 



337 



Their importance lies rather in the destruction of the slabs of beach-sandstone, in the soft 

 sand under which they burrow causing them to split off. Of more markedly tubicolous forms 

 Serpulidae and Sabellidae are conmion, but as to how far they are destructive I have no 

 evidence. 



Prof. M'lntosh^ the great authority on the Polychaeta, whose opinion on this subject I 

 requested, has been kind enough to give me the following note : — " I have not specially 

 worked at the boring of Polychaeta in tropical waters, but from casual observations think 

 that their action is not less than in European waters — probably more. Their action on 

 calcareous rocks is very conspicuous even on our own shores — especially in the south. They 

 form one of the most destructive agencies in wearing away those rocks — just as they do in 

 the case of dead shells on the bottom of the ocean — ubique." 



" Corals form a very suitable material for boring animals. I have not the slightest doubt 

 that in tropical waters the boring Polychaets, such as Poli/dora, Sabella, Dodecaria and others 

 are important agents in leading to the disintegration of rocks (especially calcareous and 

 aluminous) and of shells and of loose calcareous stones." 



Lithrotrya, a Cirripede, is a form, numerous on the seaward reefs of both Minikoi and 

 Hulule. It generally inhabits holes in overhanging masses of the raised rock of the outer 

 or seaward reefs. The animals attain a length of about 3 inches, and their holes extend in 

 for about this distance from the under surface of the ledge. In suitable positions thousands 

 may be present, riddling the whole surface of the rock. The regularity of their holes precludes 

 the idea that they can have been made by any other organism. In life they hang downwards 

 with their appendages just projecting out of their tubes, when their entrances are covered by 

 water. No other Crustacea are true borers, but Cryptochirus and Hapalocarcinus^ are of 



' Vide "On the Boring of certain Annelids,'' Ann. Mug. 

 Nat. Hist., ser. 4, 1S68, ii. p. 276 and also "Notes from the 

 Gatty Marine Laboratory. 5. On the Boring of Polydora in 

 Australian Oysters," Ann. Mag. Nat. Hist., ser. 7, 1902, lii. 

 pp. 299—308, both by Prof. M'Intosh. 



In the latter paper the author remarks that Polijdora is a 

 ubiquitous genus and well known as an active borer in shells, 

 rocks and stones. He also quotes Prof. Leon Vaillant's 

 estimate that there would be 250,000—300,000 Polydora 

 ciliata in one superficial metre of the calcareous rocks at the 

 mouth of the Somme. 



Mr Cyril Crossland, who has recently returned from 

 Zanzibar and Pemba, has favoured me with a note on the 

 Polychaeta, the group on which he has been more particularly 

 working. I do not, however, agree with some of his conclu- 

 sions, and accordingly quote him at length : — 



" The inner part and main mass of the reefs, which are 

 formed of crystallised limestone, are too hard for any boring 

 organism to touch, the only effecf I have seen produced on 

 this rock by any organism being the impression of the foot 

 and shell of a Patella. The surface of the reef-edge to the 

 depth of a few inches is soft, and this is much bored into by 

 Eunicidae, the mud and sand they produce being matted 

 together in this same position by filamentous algae. This in 

 spite of its softness forms to some extent a protection to the 

 reef edge, so that it effectively resists the action of the sea. 

 The recently formed rock of the boat channel is also much 

 bored, but not sufficiently to make any great diilereuce to 

 its cohesion. 



" Polychaeta bore more especially in living coral, one or 

 two specimens being found in most coral blocks when broken 

 open. I do not think their effects are of much importance 

 in Zanzibar in comparison to those of parasitic and aulophytic 

 sponges. However weakened the coral may be at its base, it 

 will iu general remain in its position of growth. 



" The bulk of my collection of Polychaeta was found iu 

 sand, or among weed and sponge. Their most important 

 effects lie in the transformation of moderately coarse sand 

 into fine mud. They are rare in very fine sand or in thin 

 mud, seeming rather to require, to be present in any abund- 

 ance, a fairly stifi mud or a sand coarse enough to cohere 

 somewhat. In such positions their numbers are enormous 

 (though their size is in general very small), and the amount 

 of fine mud produced by them every year must be consider- 

 able. Wherever the sand is broken by the spade one sees 

 innumerable, fine, red threads stretching across the crack, 

 these being the bodies of small Lumbriconereidae and Capi- 

 tellidae. Small tubeless Terebellidae also occur, but larger 

 forms, Eunicidae and Scoleciformia. are not so abundant. 



" The principal boring family in live coral is the Eunicidae, 

 the commonest species attaining a length of about a foot and 

 making an intricately twisted boring of corresponding length. 

 Sabellidae make short, straight holes. Serpulidae render 

 branches of Madrepores brittle by their tubes, which hollow 

 them out, sometimes every branch of a clump containing a 

 tube." 



2 Vide " Marine Crustaceans. III. Xanthidae and some 

 other Crabs," in the same Part of this Publication. 



43—2 



