62 DAVIS : BIONOMICAI. CONSIDERATIONS IN GASTROPOD EVOLUTION. 



would be subjected to torsion, besides which its efficiency would be greatly 

 reduced when it had to work round the considerable curves due to increase of 

 coiling. At the same time certain antero-lateral connections between the mantle 

 and the body-wall became of increased importance. Muscular strands running 

 from the shell through the mantle into the body -wall developed into pillars 

 going from shell to foot. They held up the shell and visceral hump when 

 the animal was extended, and served as pedal retractors. The shifting 

 branchial cavity would need to accommodate itself to the right moiety of 

 this muscular development, acquiring a position either to the- left and in 

 front of it, or to the right of and behind it. The former arrangement is the 

 one that has actually come about, probably either because of the 

 still considerable importance of external symmetry, or because of the 

 inconvenient width assocciated with a lateral branchial cavity. At all events 

 the branchial cavity became anterior, and was bounded on either side by a 

 shell -muscle, as we may now call the pillar-like structures to right and left. 



In the shift of the branchial cavity round the right side, from a postero- 

 ventral to an antero-dorsal position, what was at first on the right became 

 secondarily left, and what was primarily left acquired a position on the right. 

 The important right ctenidiuni of the primitive Gastropods thus became 

 shifted over to the left front, and most existing species possess this one only, 

 its fellow having disappeared. An exception to this is, however, afforded by 

 a few very primitive types, which have evolved on quite special lines, as 

 primitive types and survivals so often do. 



The muscular pillar on the left side of branchial cavity — in its new 

 anterior position — would tend to obstruct the none too free entrance to 

 that cavity, just at the place where the free entry of water to the important 

 ctenidium was of prime necessity. And hence this muscle has disappeared 

 from most Gastropods, except the Fissure^/iJae, Docoglossa, Haliotis, and 

 Scissute/iii. The gradual diminution of the said muscle would involve the 

 sagging of the hump on the left side, increasing llie slight tilt in this 

 direction already mentioned, so that the outer whoil would go back in an 

 almost horizontal plane, instead of a nearly vertical one. 



Meanwhile the habit of retraction became moreand more accentuated, 

 and it was perfected by the substitution of a single shell-muscle taking origin 

 fairly far back in the shell, as against a pair of muscles attached near its 

 mouth. The evolution of the habit was also associated with a lengthening 

 of the shell-cavity. This of course meant extra coiling, and if the coils 

 remained pretty much in one plane the shell would acquire the form of a 

 broad flat spiral, |)roiecting in an inconvenient way on either side ot I he 

 nnimal. It is, therefore, intelligil)le that variations in the direction of a con- 

 ical spiral should have been selected, ami wh\- the shell of an average existing 

 Gastropf)d should 1 e of this shape. In il the centre of gravity is near tiie 

 median plane, and as the morfihological (pre-torsional) right side ol the slull- 

 cavity remains central throughout, the shell-muscle is attached as it were to 



