Gastropod Foot and Branchial Cavity. 519 



stratum. PaJudina is also subject to the same danger, 

 though in a less degree, because it inhabits freshwater streams 

 and ponds. These forms have different methods of re-attaching 

 themselves after being separated from the substratum. 



Haliotisj when knocked away from attachment to the under 

 side of a stone, uses the back end of the foot in attempting to 

 re-attach itself. This part of the foot is moved about, usually 

 several times on each side of the shell, until it comes in 

 contact with the substratum. This back end and either side 

 of the foot are brought into contact with the surface of the 

 rock or stone &c, and the muscle-fibres perpendicular to it 

 contract. This contraction brings a fresh part of the foot- 

 sole into contact with the rock, and the muscle- fibres perpen- 

 dicular to it contract and bring still more of the foot-sole into 

 contact, and so on. Finally, the shell and visceral hump are 

 pulled up, and the normal position is regained. The animal 

 can re-attach itself by means of either side of the foot, 

 because, owing to the reduction of the spirally-coiled con- 

 dition of the visceral hump, the weight is practically symme- 

 trical in distribution. 



When disturbed, Trockus, Cyprosa, and Littorina retract 

 into the shell, but, being active forms, they soon push out of 

 the shell, examine the surroundings with the tentacles (in the 

 case of Trochus, if under water, with the epipodial tentacles 

 first), and then move the foot about to find a suitable object 

 on which to fix themselves. These three genera and Palu- 

 dina re-attach themselves in essentially the same way as does 

 I/aliotis, except that the front end and right side of the foot 

 are used. In the case of Trochus, Littorina, and Paludina, 

 the back end of the foot is not available for this purpose, 

 owing to the presence of the operculum, while Cyprcea has 

 descended from operculate forms. Also, probably, the habit 

 of using the front end of the foot is correlated with the 

 arrangement of the visceral hump, and it is the front part 

 which is most richly supplied with blood and possesses the 

 chief sensory developments and the most important mucus- 

 glands. 



5. Notts on the Ctenidium in the above Genera. 



The ctenidium of Cyprcea represents the ordinary Pectini- 

 branch type, possessing only one series of leaflets, though in 

 this genus the curving of the ctenidium is a modification to 

 the peculiar form of the branchial cavity. The shape of the 

 leaflets in C. pyrum differs from that in C. europcea. In 

 C. pyrum the leaflet is long and rather narrow, the afferent 



