BIONOMICAL CONSIDERATIONS IN GASTROPOD 

 EVOLUTION 



By J. R. AINSWOKTH DAVIS, M.A., 



Professor of Zoology in the University College of Wales, Aberystwyth. 



A\'ithout pre-juclging the question of MoUuscaii affinities, or speculating 

 in detail on the characters of the '' Archi-MoUusc," there seems good reason 

 for believing that this was a fl.ittened, fairly elongated, creeping type, 

 unsegmented, and probably devoid of an extensive coelom. The creeping 

 habit would be associated withatendency to increased muscularity of the ventral 

 body-wall, while — as a protective adaptation — the dorsal integument would 

 be more or less strengthened by calcareous secretion. Whatever may have been 

 the exact nature of the " prae-archi-moUusc '' it almost certainly respired by 

 the general surface of the body, and as the gradual specialization of both 

 ventral and dorsal surfaces in the manner indicated must have involved a 

 reduction in lespiratory efficiency, it is easy to conceive the pari 2>assu 

 develo[)ment— by tlie selection of favourable variations — of dorso-lateral 

 folds of integument, as a means of compensation. We are thus enabled to 

 construct a plausible h}-pothesis of the way in which the inception of two 

 primary Molluscan characters — i.e. muscular ventral body-wall and 

 skeletogenous dorsal integument —naturally led to the acquisition of a 

 third characteristic feature — the possession of a mantle. The space roofed 

 over by the mantle flaps would be the mantle cai:ity, part of this being 

 destined to deepen into a l>rawMal cavity at a later stage in evolution. 

 From some such type as that partially described it is easy to derive the 

 Placophora (Chiton^ <S:c.) by further specializations; and perhaps the 

 Aplacophora { I'loneomenia., Ntoinenia, Chaetoderma, tS:c.) may have arisen 

 from an earlier stage in the evolutionary history of such a type, though the 

 possibility of degeneration must here be taken into consideration. 



The lines upon which Chiton has evolved have evidently been determined 

 b\' the habit of clinging to stones, and creeping slowly over their surface to 

 browse upon adherent algae. The comparative length of the foot suggests 

 that it has to do a go(jd deal of creeping, and the observation of living 

 Chitons in aquaria proves that this is actually the case. It is with the 

 upward transitions from the Chitoi-type that this article proposes to deal. 



Those who have watched the slow progress of a Chiton^ and compared 

 it with the relatively agile Periwinkle ( Littovina littorea), cannot fail to 

 have realized that the more primitive animal is undoubtedly delaxed in its 

 movements by the fact that shell, viscera, and foot, are all practically 

 co-extensive for the entire horizontal extent of the body. And we can 



