518 Miss Hilda M. Bishop on the 



two lateral folds and the hack part curled forwards in retrac- 

 tion (fig. 2). In Littorina far less folding of the surface and 

 more contraction takes place (fig. 4). The reduction and loss 

 of the epipodium is probably partly correlated with the more 

 complete development of the habit of retraction in the 

 Monotocard Prosobranchs. 



The mode of contraction is also correlated with the 

 arrangement and character of the nerve-supply. 



In Trochus the longitudinal cords are obviously a drawback 

 to contraction of the creeping-surface in the middle line, since 

 nerve-tissue is non-contractile. Hence the surface of the 

 foot is thrown into folds (fig. 3). In the species in which 

 the foot is folded from back to front, this would render the 

 nerve-cords liable to injury if they were not protected in 

 some way from contact with the contracting muscle-fibres ; 

 therefore the cords are packed in very loose connective tissue 

 full of blood-spaces, and a large blood-space above and 

 between them serves as a cushion when the foot is folded 

 from front to back. There are oblique muscle-fibres crossing 

 below the blood-space and between the nerve-cords which 

 are a further protection to the latter. 



In Paludina the nerve-supply of the foot is centred in a 

 pair of pedal ganglia and a pair of longitudinal nerve-cords. 

 To secure the ganglia from injury during retraction they are 

 packed in connective tissue, and a large blood-space runs 

 between them from between the pedal cords. Besides this 

 central channel, the pedal cords are further protected by 

 being each surrounded by a small blood-space. 



In Littorina the pedal cords have disappeared and the 

 nerve-cells supplying the foot are concentrated in the pedal 

 ganglia. This greatly simplifies the question of retraction, 

 for concentration of the surface is possible to a far greater 

 extent than in Trochus. Freer disposition of muscle-fibres 

 in the vertical plane is possible, and these, by their simul- 

 taneous contraction, pull up the whole foot. The result is 

 that no great folds are produced, as in the case of Trochus, 

 though a certain amount of wrinkling of the surface takes 

 place. 



4. Re-attachment. 



Forms such as Patella, Trochus, and Littorina, living on 

 the exposed surface of rocks and weed, and Ilaliotis and 

 Cypraa, which creep on the under side of boulders in the 

 wave-disturbed zone, are subject to a constant danger of 

 being knocked away from their attachment to the sub- 



