Gastropod Foot and Branchial Cavity. 517 



3. Protection and Retraction. 



In many primitive Gastropods protection is secured by 

 bringing down the shell over the animal, e. g. Patella and 

 Ualiotis. The shell-muscle in the most primitive condition 

 is paired, but in Patella the two shell-muscles have lengthened 

 posteriorly and have fused to give a horseshoe-shaped 

 muscle, and in Ualiotis the right one has developed con- 

 siderably and has become central in position, while the left 

 one lias become extremely reduced. In Gastropods which 

 have the visceral hump and shell coiled into a spiral, protec- 

 tion is secured by retraction of the animal into the shell. In 

 the case of forms possessing an operculum, the protection is 

 more complete than in those which have not this structure. 



Pleurotomaria beyrichii, one of the most primitive coiled 

 forms, possesses a horny area, an imperfect operculum, on the 

 upper surface of the back end of the foot (Woodward), which 

 may serve as a protection against the friction of the shell and 

 against exposure when creeping. 



The method of retraction of the foot is different in the 

 various genera. 



Cyprcea possesses no operculum ; it is a descendant of 

 operculate forms which have lost this structure since the 

 establishment of a unilateral columella!' muscle. In retrac- 

 tion, the foot is folded from side to side, the middle longitu- 

 dinal line of the foot being drawn up by contraction of the 

 muscle-fibres from the shell-muscle to the creeping-surface 

 (PI. XII. fig. 1). This is probably correlated with the long 

 narrow shape of the shell-mouth. This method of retraction 

 involves very little danger of injury to the longitudinal pedal 

 cords where these are present ( C. pyrum) , and therefore packing 

 arrangements are much less elaborate than in Trochus, where 

 much transverse folding of the foot takes place. In some 

 cases, e. g. C. pyrum, the back end of the foot is folded 

 downwards and forwards in retraction — which perhaps . 

 indicates incomplete adaptation to the loss of an operculum. 



In Trochus the sole of the foot is folded considerably in 

 retraction, the principal fold being in a transverse direction 

 in T. umbilicaris and T. lineatus (tig. 3), and in a vertical 

 direction in T. zizyphinus, Cyprcea, and Ualiotis, where the 

 foot is folded from side to side. This folding of the foot 

 ■would be liable to injure the longitudinal pedal nerve-cords, 

 and each is surrounded by a mass of loose connective tissue 

 which protects it from contact with the contracting muscle- 

 fibres. In Paludina the front part of the foot is folded in 



