DEMINERALIZATION MECHANISM OF BORING GASTROPODS 83 



ABO secretion and by acids snggests, not necessarily that the secre- 

 tion is acid, but that alteration of shell surface bv the two solutions 

 is caused primarily by action on the crystalline portion of the shell. 

 The consistent uniformity and smoothness of gastropod bore holes 

 has excited much interest. Results of studies reported in this paper 

 and others in progress suggest that this uniformity is related to the 

 interrelation of the morphology and functioning of the ABO and 

 the proboscis tip. The secretory cap of the muricid ABO is located 

 on a mushroom-shaped organ with a relatiyely long thin-walled 

 stalk, and under hemostatic pressure the organ is extended fully and 

 firmly from its sac in the mid-yentral anterior region of the foot into 

 the bore hole. This brings the secretory epithelium of the ABO into 

 close contact with the bottom of the hole and confines the actiye 

 agent principally to shell immediately under the ABO disk, produc- 

 ing etchings similar to those obseryed in in vitro ABO-substrate 

 preparations. Close appression of the thin walls of the stalk of the 

 ABO to the sides of the bore holes blocks diffusion of most of the 

 secretion to these walls. It is improbable that sea water moyes into 

 the bore hole as the snail alternates use of the ABO and the radula, 

 sijice the hole is completely coyered by the foot while the ABO is 

 functioning, and in shifting to use the radula the snail extends its 

 proboscis to the boring site between closely juxtaposed propodial 

 folds. But if sea water does enter, it could be pressed readily from 

 the hole by full extrusion of the ABO into it. The snail rasps prin- 

 cipally at the bottom of the hole, rotating the radula to effect uni- 

 formity in penetration. Zones or areas of "harder" shell occasionally 



Figs. 38 to 43. Effects of secretion from excised ABO on pure mineral 

 crystals. 



Fig. 38. Optical micrograph of etching of ABO of Eiiplcuia cciudaia 



etterae, 3-hour exposure, on calcite. (x 30.) 



Fig. 39. Electron micrograph of normal calcite surface. ( X 6000.) 



Fig. 40. Electron micrograph of part of etched surface shown in Fig. 38. 



(X6000.) 



Fig. 41. Optical micrograph of etching of ABO of Urosdipinx cincrca fol- 



hjciisis after 3-hour exposure on aragonite. ( X 30.) 



Fig. 42. Electron micrograph of normal aragonite surface. ( X 6000.) 

 Fig. 43. Electron micrograph of part of etched surface shown in Fig. 41. 



(X6000.) 



