LOCOMOTION OF BERMUDIAN MOLLUSKS 235 



the trials contact with the plate did not cause the posterior end 

 to become attached. 



The only other mollusk I was able to cause to move backward 

 was the keyhole limpet Fissurella. When the posterior fourth 

 or more of its foot was attached to the lower edge of a vertical 

 glass plate, the head of the animal being directed downward, 

 the first wave or two would carry the limpet straight backward 

 for a distance of 2 millimeters or more, then turning would take 

 place. Here again the direction of the wave which carried the 

 animal backward was the same as that which carried it forward. 

 All other mollusks tried were so flexible or had such a large foot 

 that they would bend over and attach the anterior part of the 

 foot to the other side of the glass. Naturally conditions under 

 which these experiments were conducted would hardly be 

 realized in nature. Backward locomotion is probably seldom 

 resorted to by gastropods, at least for any considerable distance, 

 since turning took place sooner or later in every trial. Never- 

 theless certain mollusks possess the ability to perform such 

 movements. 



A clear demonstration that pedal waves are concavities and 

 not convexities (Biedermann, '05; von Uexkiill, '09; Parker, '11) 

 is afforded by the use of a manometer. A hole 1 mm. in diameter 

 was bored in a glass plate. Under this was fastened by an air- 

 tight joint of "orange sticky wax" one end of it capillary tube 

 which was bent back on itself in two places forming a letter V. 

 The tube was marked off in millimeters and a colored solution 

 was introduced so that any change in level could be readily 

 detected. This capillary manometer was used by placing a 

 mollusk on the underside of the plate and watching the liquid 

 as the animal passed over the hole. For every gastropod tried — 

 Cypraea, Veronicella, Onchidium, Tectarius, Fissurella, Eulota, 

 and also Chiton — the liquid was drawn toward the animal just 

 as a wave passed over the opening in the plate, and then returned 

 to its former level immediately after the wave had passed. This 

 shows that the wave exerts a suction and must therefore be a 

 concavity. The Chiton foot seemed to give the greatest amount 

 of suction of all the mollusks tried, a difference in level of 3 to 4 



