CONTROL OF CORAL REEFS BY BORING SPONGES 33 



served the removal of calcareous chips bv recently settled sponge 

 larvae before spicules had developed. 



Topsent ( 1888 ) believed that a purely mechanical action brought 

 about by the contractility of the sponge cells was sufficient to ex- 

 plain the destructive activities of clionids. He argued that dissolu- 

 tion of the calcareous matter by an acid cannot be involved for 

 two reasons: (1) conchiolin is removed in the form of small chips 

 similar in size and shape to the chips removed from calcareous mat- 

 ter; and (2) the sponge cells in contact with the substrate differ in 

 size from the calcareous chips removed, although he believed that 

 the surface area of the chips should follow the outlines of the acid- 

 secreting cells. In answer to Topsent's first point Cotte ( 1902 ) , 

 Nassonov (1924), and Vosmaer (1933-1935) have suggested that 

 an enzyme may be secreted which attacks the organic components 

 of mollusk shells. The observations of Nassonow ( 1883 ) on post- 

 larval clionids indicated that protoplasmic processes of the sponge 

 cells are the site of the boring activities of the sponge; thus the 

 size of the cells need not correspond to that of the chips. Nassonow 

 also noted certain elongate protoplasmic strands extending from 

 adult clionids into the calcareous substrate, and thought that these 

 were responsible for the extension of the excavations made by the 

 sponge. Topsent (1887) pointed out that these latter protoplasmic 

 prolongations are in reality boring algae or fungi and are quite in- 

 dependent of the sponge and its activities. He stated that these 

 organisms are of common occurrence in dead bivalve shells whether 

 they harbor boring sponges or not. It was remarked by Cotte 

 (1902), however, that, although Nassonow was in error in his inter- 

 pretation of the elongate strands seen extending from adult sponges, 

 he was correct in his observations of cell processes extending into 

 the calcareous substrate in his postlarval preparations. In a study 

 of decalcified sections of Cliona vastifica boring in a mollusk shell, 

 Cotte saw pseudopodial expansions of mesenchymal cells extending 

 beyond spaces between the outer epidermal cells of the sponge 

 where these are in contact with the substrate. He noted that these 

 mesenchymal cells are identical with the spherule-containing cells 

 described by Topsent ( 1900 ) , stating that he could see granules in 



