BORING AND SAND-FEEDING ORGANISMS. 335 



2. Boring Organisms. 



Nearly all the skeletons of the living reef corals in the Maldives are riddled with boring 

 sponges and algae. The algae all belong to one genus, probably Achyla, and seemingly to a 

 single species. The sponges are of two kinds, those which form large spaces in the coral 

 skeletons, which they themselves fill, and those whose presence is more or less imperceptible. 

 The latter belong to the genus Cliona and probably like the algae to a single species. Achyla 

 and Cliona resemble one another in their mode of growth. Their ramifications are most 

 delicate, imperceptible to the unaided eye, and wander all over the coral skeleton. In Pocillopora 

 their terminal filaments extend so close to the ends of the branches that only the very thinnest 

 layer of corallum separates the polyp tissues from them. When a branch of this genus is 

 decalcified and the pol3rp layer carefully stripped off, either of these boring organisms will be 

 seen to have formed a close-meshed network, showing accurately the shape of the original branch 

 with all its twigs. 



The two genera do not in the Funafuti or other Pacific Ocean corals occur together, but 

 whether they do so in those from the Maldives I cannot as yet say. To separate the two 

 organisms a minute microscopic examination is required, but the presence of one or other 

 in most reef corals is assured. The importance of these forms lies in the fact that they riddle 

 the coral skeleton as soon as ever it is laid down. In the dead or decaying portions of 

 coral masses they are not found ; indeed, they seem generally themselves to die with the coral 

 they inhabit. Of themselves they do not, so far as I have seen, directly cause decay, but 

 presumably they show the way to other boring organisms, which certainly are not slow to follow. 

 Their importance, indeed, is not improbably extremely great, but as yet little or nothing is 

 known of their life histories, or modes of growth. 



A second kind of sponge — apparently a Myxospongid — is of some importance for its action 

 on corals, particularly within the lagoons of atolls. It apparently enters such coral colonies 

 as are more or less dead at their attached ends — thus presenting bare surfaces not covered 

 with polyjDS or epitheca — and hollows out cavities up to a square cm. in size or even larger, 

 which it at once fills up with a mass of yellow or grey sponge. From one space it sends on 

 its growths and excavates other cavities, thus perhaps completely riddling the base of the 

 mass and causing it to fall. No connection with other boring animals was in any case observed 

 or indicated, and the presumption necessarily is that it forms its cavities by the action of some 

 acid secretion. The corals most affected by its growth were perforate forms, particularly 

 Madrepora, but I also found it in many Astraeidae as well. An intei'esting point lies in the 

 fact that this sponge seems to become more prevalent with increase of depth down to 50 

 fathoms, beyond which I do not know of its occurrence. 



Of Mollusca Lithodomns (Mytilidae) is often very destructive, as it bores large, perfectly 

 rounded holes up to 12 or 13 mm. in diameter through the coral masses. These often extend 

 for several inches or even a foot or more straight through a colony, and, where one is found, 

 there are usually a great number. The various holes seldom or never communicate with one 

 another, and seem for the most part to run almost parallel. There is always a communication 

 with the exterior through some dead part of the corallum, by which the animal entered in 

 the first place, and the polyp tissues are sometimes broken through as well, the original hole 

 then perhaps being plugged up by sponge or other growth. The animal has no preference for 

 one kind of coral, but all are equally affected, the base of a Madrepore colony, the skeletons of 

 massive Porites or Astraeidae, or even a thick branch of Pocillopora. Lithodomus was extremely 



G. 43 



