6o DAVIS : BIONOMICAL CONSIDERATIONS IN GASTROPOD EVOLUTION. 



likfly he seen the first heginning of the swimming-habit, which is now so 

 characteristic of Cephalopoda. The long axis of the visceral hump was 

 already directed postero-dorsally away from the head, and the body would 

 tend to dispose itself symmetrically about this incipient swimming-axis, in 

 the most convenient manner for swimming. Extensions of the body 

 diverging from the axis would gradually diminish, and this enables us to 

 understand the disappearance of the foot-sole, with concomitant hypertrophy 

 of the vential musculature below and around the mouth. It is very probable 

 that there were lateral muscular extensions, possibly of use for parachuting, 

 and most likely also of sensory value. These extensions have persisted in 

 the Cephalopods as the funnel, and in many Gastropods as epijxxiia. 

 It is interesting to observe that the " parapodia " of Aplysi i — regarding the 

 morphological value of which no opinion is here expressed — are lateral 

 muscular flaps used both for parachuting and swimming. The main lines 

 of adaptation — as regards external form — have now been briefly sketched out 

 for the prototypes of the Cephalopoda, and a similar attempt will next be 

 made for the Gastropoda. It is necessary to begin by an account of the 

 probable general lines of evolution of the branchial cavity. 



The blood-stream of primitive Molluscs flowed forwards in the gut-wall 

 to the head— the most important region — so that a deficiency of blood was 

 continually being created just beneath the poster(,)-dor.sal integument, to 

 which there would, therefore, naturally be an inflow from the other parts of the 

 body. For this reason the posterior integument on either side of the middle 

 line — in the region of the rectum and anus — early acquired special 

 respiratory value, though the present writer does not attach any particular 

 importance to the exact homologizing of respiratory outgrowths in different 

 types. At any rate, as general width was reduced as the outcome of a more 

 active habit, there was a specialization of such outgrowths at the back, and 

 one or two gills (ctenidia) increased in size, that pait of the mantle-cavity 

 shelterint'; them deepening pari ini>^m, and liecoming a trui; Uranrlilal rariiij. 



The anus was median and posterior, remaining so notwithstanding the 

 development of a branchial cavity. The incurrent respiratory water-streams, 

 coming in on the sides iKithed the ctenidia, and then coalesced into an 

 excurrent stream, flowing past the anus, and flushing the branchial cavity. 

 \\'hatever the exact original position of the excretory openings, it was at any 

 rate more or less posterior, and not very far from the anus. By .selection of 

 variations these openings were gradually shifted back towards the median 

 plane, i.e. nearer the excurrent stream, by which their deleterious products 

 were washed away. It is characteristic of ('e])halopods that the left side is 

 of predominant importance as regards sexual products, i.e. the one gonad 

 when — as in most cases — only one exists, is the Ifft. The same is true ior 

 Gastropods, where the gonad primitively expels its products through the 

 morphologically left kidney, so that the excretory outlet of the gona<l- 

 posscshing side, i.e. the left, must have been of predominating impoitance. 



