NUTRITION AND METHODS OF FEEDING 195 



The whelk {Buccinum), dog-whelk {Purpura) y and forms 

 such as Nassa and Natica are carnivorous, feeding either on 

 decaying animal matter or on living " shell-fish," by sucking 

 out their soft bodies through a hole bored in the shell. The 

 dog-whelk undoubtedly feeds on rock-barnacles with which 

 it is invariably associated although, so far as we are aware, 

 the manner of its feeding has not been investigated. The 

 neat perforations often found in empty bivalve shells are 

 caused by carnivorous Gasteropods and are usually ascribed 

 to the action of the radula. There is, however, not a little 

 difficulty in understanding how the radula, which is essentially 

 saw-like in its action and incapable of being rotated, could 

 begin its task, or how the rough teeth could produce such a 

 smooth margin to the perforation. According to Schiemenz 

 (189 1), the rough holes which Natica produces in mussel 

 shells are not drilled by the radula, the function of which is 

 in this case merely of secondary importance. The hole is 

 the work of a " Saug-platte " (suction disc), or glandular 

 organ, situated on the under side of the proboscis, which 

 secretes an acid substance (H2SO4), capable of dissolving 

 the shell. If a shell is examined before the work of perfora- 

 tion is complete a small boss is often observed in the centre 

 of a circular depression. Not only does the concavity 

 correspond in size with the " Saug-platte " (better called a 

 boring gland), but in the centre of the latter there is a 

 concavity corresponding exactly to the boss or convexity 

 left in the shell. After the boring process has been continued 

 for some time, the floor of the depression breaks through. 

 The perforation is more or less kidney-shaped and has 

 rough edges, which are evidence that the radula has now 

 been brought into play. The work of boring is finished with 

 the aid of the radula, the inner aperture of the completed 

 perforation being narrower than the outer and having a 

 slightly less even edge. 



In bivalve molluscs we find typically developed the 

 use of ciliary currents. This method of feeding is accom- 

 panied by an inactive life, the animal being frequently 

 buried in sand or mud as a means of escape from enemies and 



