32 T. F. GOREAU AND W. D. IIARTMAN 



markable network of thread-like interconnections and pseudopodia, 

 often 50 /x or more long. These surrounded areas reminiscent in 

 shape of the calcite particles excavated by the sponges, or of the 

 lines etched on calcite crystals." 



Warburton dissolved away the calcareous components of an oys- 

 ter shell riddled by Cliona and thus freed sheets of conchiolin, 

 a proteinaceous substance lying between successive layers of cal- 

 cium carbonate in the shell. He confirmed the observations of Top- 

 sent ( 1888 ) that the sponge is able to penetrate this substance and 

 does so by removing particles comparable in size and appearance 

 to the particles of calcite removed. Nassonov (1924) had also made 

 this observation; both he and Warburton noted, however, that the 

 sponge bored through conchiolin less readily than through CaCO.!. 

 In oyster shells the sponges excavate extensive galleries through 

 the successive horizontal layers of CaCO:-,, but penetrate the inter- 

 calated sheets of conchiolin only at rare intervals. 



Clionids thus excavate their galleries in calcareous materials by 

 a relentless removal of small chips of CaCOs. Not only are piles of 

 such chips to be seen at the base of each oscular opening of the liv- 

 ing sponge, but also the burrows themselves give evidence of the 

 removal of numerous pieces of CaCOs by the shagreenlike appear- 

 ance of their surfaces ( Fig. 7 ) . 



How are the chips removed? Both mechanical and chemical ex- 

 planations have been put forward, but there is no unequivocal 

 evidence for either as yet. 



Hancock (1849) supposed that the contractile powers of the cells 

 caused certain siliceous bodies, about 42 microns across and em- 

 bedded in the surface of the sponge, to grind away the calcareous 

 matter. Later he (1867) recognized that the "siliceous bodies" were 

 in reality fragments of conchiolin being excavated from the shell 

 by the sponge. He then suggested that smaller angulated siliceous 

 bodies, about 4 microns across and also embedded in the sponge 

 surface, or the spicules themselves, must effect the excavation of 

 the calcareous material. Hancock's theory (also maintained bv 

 Fischer, 1868) has now been abandoned completely, since Nas- 

 sonow (1883; confirmed bv Old, 1942, and Warburton, 1958) ob- 



