ROCK-BORING ORGANISMS O 



desiccation, and occurs only when such dangers exist. It is a purely 

 facultative habit and is found in various intertidal echinoids. Umb- 

 grove (1947) considered Echinometra mathaei to be a major agent 

 of erosion in East Indian coral reefs. 



Only when the borings become deep and finally flask-shaped are 

 the animals imprisoned within them. Usually they are able to leave 

 the "boring" and forage on the surrounding rock surface. Food con- 

 sists largely of encrusting algae, and probably sufficient fragments 

 are carried in by water movements to support imprisoned animals. 

 But any algae growing on or within the rock in the borings will be 

 a source of food. There is no question here of chemical action, in- 

 deed the rock may not be calcareous, and boring is to a large extent 

 fortuitous. The whole matter has been most thoroughly reviewed 

 and discussed by Otter ( 1932 ) . 



No Association with Nutrition 



Sponges. A variety of sponges, most notably and ubiquitously 

 the numerous species of the genus Cliona, bore into calcareous rocks, 

 into coral skeletons, and into mollusc shells. Cliona celata, which 

 bores into calcareous structures, especiallv ovster shells, in European 

 waters is probably the best-known species. The larvae settle from 

 the plankton and the developing sponge quickly penetrates the shell, 

 forming extensive galleries so that the whole structure becomes fria- 

 ble and crumbles away. There can be no question here of the 

 sponge's gaining food in this way; like all sponges, it feeds on nano- 

 plankton drawn into the body in currents created by the flagella. 

 It does not necessarily bore, indeed this would appear to depend 

 on the nature of the substrate on which the larvae settle. The method 

 of boring remains obscure; the siliceous spicules could assist mechan- 

 ically, but, in view of the calcareous substrate, acid could obviously 

 be used. Growth processes could also assist, as suggested in the case 

 of algae. Indeed, as noted by Gardiner (1903), the algal Achijla and 

 Cliona resemble each other in their mode of growth; Gardiner adds 

 that "their ramifications are most delicate, imperceptible to the un- 

 aided 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 



