48 



latter over the tongue they point upwards. It is not easy 

 to see how they could be of much use in boring if they 

 remained in this position. One sees here perfectly clearly 

 the reason for the powerful dorsal musculature. When 

 the radula is pulled by the ventral muscles the teeth slip 

 backwards over the object with little friction since they 

 are pointing the other way, but when the dorsal muscles 

 contract, the teeth are directed against the object and 

 much resistance has to be overcome. 



The action of the radula can, as a matter of fact, be 

 easily demonstrated in whelks that have been narcotised. 

 If the proboscis is removed from one of these animals and 

 squeezed between the finger and thumb gently, the 

 odontophore will be protruded, and the application of a 

 little pressure to a spot which can be found by trial will 

 cause the radula to move backwards and forwards over 

 the cartilage. 



The action of the muscles, so far as I have been able 

 to determine, is as follows : — By contraction of the dorsal 

 retractors, the radula is pulled so that the teeth rasp the 

 object. At the same time the occlusor muscles come into 

 action and hold the sides of the cartilage together, so as 

 to maintain the groove. I find no evidence whatever to 

 show that by contraction of these muscles the radula 

 comes to lie on the sides of the cartilages as stated by 

 Oswald. On the other hand, the position of the lateral 

 teeth show the necessity of the groove, for as they pass 

 from the plane ventral surface of the tongue to the gutter- 

 like dorsal surface, they " bite " inwards, and so the net 

 result is a tri-partite attack on the object. 



The ventral muscles now come into play and draw 

 the radula easily backwards. There would be no point 

 in the arrangement of the lateral teeth whereby they can 

 rotate inwards and act in a most efficient manner, if 



