276 DE. H. J. FLEUEE ON THE EVOLUTION OF 



variations of reduction of tlie latter. The mantle-edge, always near the shell-edge, 

 would also, under the circumstances, perform most efficiently that function of giving the 

 animal an impression of its immediate surroundings which had been the raison d'etre 

 of the epipodium. We therefore understand the reduction of the epipodium and the 

 development of sensory structures on the mantle-skirt. The edge of the latter was 

 already occupied by shell-secreting glands, so the sensory zone was differentiated ventral 

 to this. 



No trace of the epipodium exists among Monobranchs, but a lateral glandular streak 

 occurs along the sides of the foot in Patina, among the Nacellidge, and in very young 

 specimens of Patella. This structure has been homologized with the epipodium by 

 Pelseneer, but the homology is denied by Haller because of the non-development of the 

 streak among the Monobranchs, which he considers to be the ancestral group. Thiele, 

 too, denies the homology, though he looks upon the Cyclobranchs as the more primitive 

 group ; his opinion rests on the view, which lie alone adopts, that the epipodium is 

 no part of the foot, but rather the equivalent of the " Notaum " of Chitonidne, the organ 

 which surrounds and secretes the shell-plates, and is by other specialists homologized 

 with the mantle of Gastropods. In any case, there is no doubt that the epipodium 

 has been very much reduced among the Docoglossa, and this is all that concerns us at 

 this stage. 



As the adaptation of the shell-edge to the form of the underlying surface became 

 important, and the available space was limited by competition of barnacles, algre, &c., 

 the possession of a " home " became a valuable consideration. Since the animals 

 still crept about to some extent, residence on one spot necessitated the evolution of the 

 " Homing-Power " which is such a well-known and remarkable feature of Patella. 

 The sensory arrangements which make homing possible have been the subject of 

 discussion, and Lloyd Morgan has credited tlie cephalic tentacles with this function. 

 Ainsvvorth Davis has, however, found that Patella homes after excision of the cephalic 

 tentacles, and he ascribes the homing faculty to the pallial tentacles. In his favour is 

 to be said that the mantle-tentacles become differentiated among the Docoglossa (PI. 17. 

 tig. 21 a-c), they are undeveloped in Acmcea virglnea, small and fairly equal in^. testu- 

 dinalis, and very numerous in Patella and most Cyclobranchs, where they are divided 

 into large and small tentacles. The concomitant development suggests correlation, 

 which is also otherwise probable ; too much stress must, however', not be laid on this, as 

 the pallial gills have also developed concomitantly Avith them, and the mantle-tentacles 

 may be correlated with protection of these latter from unfavourable influences. 

 " Cyclobranchs often possess a depressed scar, in regard to which I agree with those 

 observers who think it due to wear and tear, shuffling of the foot, &c. The edge is the 

 deepest part of the scar, no doubt as a result of friction with the shell-edge. This 

 depressed edge of the scar possesses a special biological value, for it places the shell- 

 edge, the animal's vulnerable point, in a groove from which it is all the more difficult to 

 dislodge it. 



As Haller and others have stated, the eyes have degenerated, because the head always 

 remains under the shadow of the protecting shell. 



