Tarr (1885) in a superficial description of the structure of the proboscis 

 gave a brief preliminary account of some of the aspects of the mechanical phase 

 of drilling, By the use of a perforate cupped oyster valve containing a living 

 shucked oyster and sandwiched between two microscopic slides immersed in a 

 finger bowl of sea water with a few hungry drills (PrytherCfl's technic, unpub.), 

 it has been possible more recently to observe the precise drilling behavior of 

 U. cinerea under the binocular microscope (Carriker, 1943; and later, unpub.). 

 When placed on living oysters some drills select the drilling point quickly and 

 others search for some time before making the choice. During the search the 

 proboscis is extended and its tip, undulating with minute wave -like movements, 

 is passed slowly over the substratum. Using Prytherch's technic the writer soon 

 discovered that drilling frequently is performed at the junction of the shell and 

 one of the glass slides. The proboscis tip is extended to this site apparently 

 attracted by chemical stimulation from the living oyster within, and since the glass 

 slides partially bar the entrance of the snail shell, the proboscis and drilling are 

 clearly visible . As drilling progresses it is soon evident that the radula is only 

 very slightly effective in rasping through the calcareous layers of oyster shell, and 

 that penetration is made possible through the activity of the accessory proboscis. 

 After the drill has rasped the drilling site free of incrustations, penostracum, and 

 soft shell material, it withdraws the proboscis and creeps forward until the accessory 

 proboscis comes to lie immediately over the drilling site . The foot during this time 

 adheres very strongly to the prey (this may account for Fretter's interpretation of 

 the use of the gland); the accessory proboscis billows outward and completely fills 

 the hole, remaining in this position for several mmutes. During this time no 

 noticeable movement of any part of the snail is evident, and the ventral surfaces 

 of the foot remain tightly applied to the shell surrounding the hole, so that a 

 watertight connection seems to be maintained After a time the accessory 

 proboscis is gradually withdrawn, the anterior part of the foot is backed away, and 

 the proboscis is protracted, It soon locates the drilling site and continues rasping. 

 This alternate process is repeated until the shell is perforated. Rasping intervals 

 vary from 2 to 15 mmutes and the alternate periods of softening, one to 47 minutes 

 Some unidentified chemical secreted apparently by the accessory proboscis appears 

 to soften the shell material. This is suggested by the fact that after each softening 

 period the radula removes microscopic flakes of the shell material which during 

 the latter part of the previous rasping interval did not respond to rasping. It is 

 possible, as suggested by T. C. Nelson (pers. com.) that secretions of the accessory 

 proboscis acting on the conchiolm matrix of shell, free crystals of calcite, calcite- 

 ostracum, and chalky deposits in the shell (also see Galtsoff. 1954). Bits of shell 

 material removed by the radula are carried back into the buccal cavity where suction 

 from the esophagus draws them off the teeth and passes them into the stomach. The 

 translucency of the proboscis permits observation of these functions . The frequency 

 of the rasping strokes in an adult drill at 25° C was about 60 per minute 



24 



