34 T. F. GOREIAU AND W. D. IIARTMAN 



the pseuclopoclia like those in the cell body. Cotte agreed with 

 Nassonow in ascribing to these cell processes the role of dissolving 

 awav the calcareous substrate in such a wav as to remove small 

 chips of CaCO:;. As noted above, Warburton (1958) also observed 

 a network of pseudopodia extending from clionid cells in contact 

 with the substrate. 



Letellier (1894) supposed that the contractilitv of the cells adher- 

 ing to the substrate brings about sufficient pressure to break awav 

 small particles of CaCOa. He cemented small threads or rods of 

 rubber and gutta-percha to oyster shells and found that by twisting 

 these in various directions he could eventuallv break awav chips 

 of shell with shapes similar to those removed by clionids. 



Nassonow (1883), Cotte (1902), Vosmaer (1933-1935), and 

 Warburton ( 1958 ) have all proposed that the chips are removed 

 bv a localized dissolution of calcareous material at the point of 

 contact between the cytoplasmic extensions of the cells and the 

 substrate. As yet no investigator has demonstrated a lower pH at 

 these points of contact, and there is no clear-cut evidence showing 

 an increase in dissolved calcium in media in which clionids have 

 been cultured (Old, 1942; Warburton, 1958). Vosmaer (1933- 

 1935) has suggested that carbonic acid produced by the sponge is 

 the substance most likelv to be responsible for the dissolution of 

 the calcareous substrate. 



In summary, it may be said that attempts to explain the boring 

 activities of clionids in terms of mechanical abrasion by spicules 

 have been ruled out bv the observations of Nassonow (1883), Old 

 ( 1942 ) , and Warburton ( 1958 ) on postlarval sponges lacking spic- 

 ules. That the adhesive and contractile properties of the sponge 

 cells alone can bring about the excavations seems unlikely as well. 

 Cotte ( 1902 ) has pointed out that in old shells weakened bv a net- 

 work of algal borings, if forces of traction were exercised b\' clionid 

 cells, the chips broken away would be irregular in shape. Instead, 

 in such instances, the lunules of CaCO.i removed from the substrate 

 are identical with those which occur when the sponge bores into 

 calcite. Warburton (1958) has noted that, if mechanical action of 

 the type described is involved, it would be expected that the chips 

 would be severed along planes parallel to the cleavage faces of 



