HOCK-BORING ORGANISMS "21 



pecially posteriorly) when the valves are open. The anterior glands 

 are large only in Lithophaga, where alone the tissues in which they 

 are situated can be protruded. The granular layer on both anterior 

 and posterior shell surfaces in Lithophaga and over posterior regions 

 only in Botula consists of calcareous fragments and other debris 

 (only the latter in Botula) which are consolidated in mucus secreted 

 by these glands ( Yonge, 1955 ) . The presence of acid in the anterior 

 glands — and probably also in the posterior glands, which may assist 

 in the widening of the boring — is confined to Lithophaga. 



Some species of Lithophaga rotate in the boring, e.g. L. phimula 

 (Yonge, 1955); others do not, e.g. L. cumingiana (Otter, 1937). 

 In the majority of species — there is some degree of variation within 

 the genus — the posterior region of the boring has a secreted cal- 

 careous lining, resembling that described in RoceUaria although 

 not so well developed. This is produced by the walls of the siphons, 

 here consisting of the inner lobe only of the mantle margin, the in- 

 halant siphon being produced by apposition, not fusion, of the 

 ventral margins. In certain species, e.g. L. phimula, the siphons 

 may also be responsible for adventitious posterior extension of the 

 shell valves. 



Although Botula and Lithophaga doubtless descend from com- 

 mon ancestors, conditions in the former, exclusively mechanical, 

 borers foreshadow those in the latter. With the development of 

 the means of chemical softening of the rock, Lithophaga has lost 

 full efficiency for mechanical boring but has exploited to the full 

 the added powers of boring into calcareous rocks of various kinds. 



Discussion and Summary 



Although, as noted in the introduction, the habit of rock boring 

 is unexpectedly widespread, this brief review indicates how many 

 of the borers are not specifically adapted for this mode of life. In 

 many cases boring is facultative, not obligatory. The condition in 

 the plant borers is somewhat obscure, but among the animals boring 

 is not obhgatory in the sponge Cliona, in the echinoids, in Polydora, 

 or, among the bivalves, in Hiatella and the less specialized species 

 of Petricola. All these animals are members of the epifauna, with 



