DAVIS : RIONOMICAL CONSIDERATIONS IN GASTROPOD EVOLUTION. 59 



understand the evolution of forms creeping with greater rapidity, by the 

 selection of variations tending to the aggregation of the viscera away from 

 the muscular foot. Hence most probably the development of the 

 "visceral hump", which plays such an imjiortant part in Molluscan 

 morphology. 



In some of the descendants of the Chiton-ty])e (not of the Chitons, 

 which of course have specialized along their own lines), the acquisition of a 

 more actively creeping habit would seem to have been associated with the 

 development of such a hump, {)ostero-dorsal in position, but as life beneath 

 stones necessitates a considerable amount of lethargic clinging, we may 

 su|i[)<)st; the hump to have been symmetrical and median. Some of these 

 forms were also probably accjuiring the widespread habit— especially among 

 (ia5tropods - of creepmg against the surfcice-film of fairly still water, with 

 their b(jdies immersed, and in such forms the symmetrical disposition of the 

 main weight of the body, i.e. of the visceral hump, would be accentuated. 

 For here again wc have a variety of the clinging habit, and as the creeping 

 organ which supports the weight is necessarily symmetrical about the axis of 

 progression, the weight itself must also be symmetrical. Otherwise a couple 

 w(juld be brought into existence, tending to turn the animal out of the 

 plane in which its foot could cling. 



Other types, probably coming off nearer the root of the Molluscan 

 evolutic>nary tree, would seem to have extended their wanderings from their 

 first home on the under side of stones to the floor of the spaces underneath the 

 boulders of the shore, afterwards leaving the surface and ploughing their 

 way into the gravel, sand and mud. We may thus picture the first beginnings 

 of the Pelecypoda, which have acquired their characteristic shape— as 

 is generally admitted — by a narrowing and deepening of the primitive 

 Molluscan body. The burrowing Scaphopoda probably arose from the stage 

 in which a postero-dorsal visceral hump had already degun to develop, and 

 the gradual hypertrophy of the anterior musculature is readily intelligible. 



We will now return to the Chiton-type in which the viscera were 

 collecting into a symmetrical postero-dorsal visceral hump. This prol)al)ly 

 possessed a protective covering in the form of a cap-shaped shell, beyond 

 which the foot stretched freely backwards, the ujjper surface of this posterior 

 extension beginning to be chitinized as a i)rotecti()n against tiu- rubbing 

 of the shell-edge, and presenting the fir.^t stage in the evolution ol an 

 operculum, thougli as yet with only one of the functions of that organ. 

 Such a tyj)e would be able to creep with fair activil), not onh under but 

 also around and upon boulders, and it would i)rol)abiy also make use of 

 surface teiisioi; for crossing fairly still tide pools. A ty|)e like this may be 

 imaginetl as corresfionding to the c(jmmon stock from which both 

 Ce])halopods and Castropods have originated. The creeping by surface- 

 tension was jirobably associated with a certain amount of para- 

 chuting when the hold of the surface film was lost and in this may \ery 



