516 Miss Hilda M. Bishop on the 



tide habits. They have acquired a habit of adhering to the 

 rock when out of water by the rim of the shell, with the foot 

 entirely retracted. This is effected by means of mucus, 

 which hardens on exposure to the air, and so cements the 

 shell to the rock. The entire retraction of the foot lessens 

 the danger of access of drier air to the branchial cavity, 

 which would harden the mucus contained in it, thus injuring 

 the gills. 



Lacuna, a member of the Littorinida?, has been seen to 

 creep against the surface-film, and to let itself down from 

 this by a mucus-thread from about the middle of the foot. 



Individuals of Cyprcea europcea kept in tanks have been 

 observed to creep along the sides of the tank above the surface 

 of the water, remain fixed for a time with the front end of 

 the foot folded back over the head-region, and then to gradually 

 lift the foot from the side of the tank, beginning at the 

 front. This continued until the animal remained fixed only 

 by the extreme back end of the foot, and finally the weight 

 became too great and it fell back into the water. In its 

 natural surroundings the animal creeps on the under side of 

 a stone or rock, and exactly the same process is gone through, 

 except that it becomes detached from the substratum and 

 remains suspended by a mucus-thread from the extreme back 

 end of the foot. The difference in the position of the mucus- 

 thread relatively to the foot in Lacuna and Cyprcea is 

 probably due to the difference in the distribution of weight 

 in Cyprcea from that in Lacuna, the visceral hump in Cyprcea 

 being coiled symmetrically over the foot, while that of 

 Lacuna is coiled on the right side of the animal ; and also 

 perhaps to the fact that the end of the foot in Cyprcea^ which 

 no longer possesses an operculum, is freer than is the case in 

 the operculate Lacuna. The mucus is secreted largely 

 probably by the anterior glandular groove of the foot. 



Cyprcea creeps about between tide-marks, especially upon 

 the under surfaces of rocks, stones, and weed. Siphonate 

 forms are commonly supposed to be purely carnivorous, but, 

 individuals of Cyprcea living in tanks have been found to 

 browse upon seaweeds, particularly Uha latissima. Their 

 habit of creeping upon the under surfaces of rocks &c. 

 probably accounts for the external sj^mmetry of the body with 

 regard to the centre of gravity, for symmetrical distribution 

 of weight would be an advantage in the maintenance of such 

 a position. 



