D.— ZOOLOGY. 87 



In any case I venture to suggest that the torsion of Gastropods arose 

 in the first place very much as you see it develop to-day, as a larval 

 adaptation, in response to larval needs ; and that it was perpetuated 

 because, once accomplished, it was of immediate advantage both to 

 larva and adult. It transformed the earlier Nautiloid type of larva into 

 the much more effective Veliger ; and the Veliger, settling down to 

 resume the benthic life of its sire, foimd no serious obstacle to growth in 

 the new arrangement. Lastly, the adult, whose head had previously had 

 little or no protection, was now able to withdraw it on disturbance into 

 complete safety. I assume that the ancestor had a muscular creeping 

 foot, neither so simple as that of Chiton, nor so complex as that of a 

 Cephalopod, but definitely suctorial, though capable of progression. 



The only mutation required to start the torsion in the assumed ancestral 

 prse-Gastropod larva was an asymmetry in the development of the retractor 

 muscles, thus bending the head and foot round during contraction ; but 

 it remains for further investigation to show whether or not the rotation 

 in Patella and Acmcea is actually determined in this way, as indicated by 

 the rapidity of its accomplishment. The ideal muscular arrangements for 

 bringing about complete rotation would consist of a right-sided cephalic 

 retractor with posterior attachment and a left-sided pedal retractor with 

 an attachment in front of the other, the two crossing one another more 

 or less at right angles. Patten's figures show that these conditions are 

 realised in the prse-torsional stage so far as the right side is concerned 

 (see fig. 4, second figure from the right), but unfortunately leave us in 

 the dark as to the arrangement of muscles on the left side. It is manifest, 

 however, that, owing to the small size of the larval body, any muscular 

 disparity between right and left sides in the direction indicated would 

 conduce towards a reversal of the relations of head and foot to the mantle- 

 cavity at every contraction, while fixation of the organs in the reversed 

 position would be a simple matter at this stage of development, when the 

 body-muscles are just beginning to be actively differentiated, and their 

 connections have yet to be established. Thus, although the theory of a 

 larval origin of the Gastropod torsion cannot be established on our present 

 data, we can at least claim that rotation may have been accomplished in 

 the way suggested, and that the ease and rapidity with which it could be 

 achieved contrast favourably with the difficulties besetting any theory 

 of progressive torsion through a long series of adult ancestors. In the 

 larva the smallest twist would produce a favourable change. 



In his own ingenious theory of 1913 Prof. Naef has suggested a novel 

 way out of some of these difficulties. He regards the ancestor of 

 Gastropods as a free-swimming Mollusk, not unlike a small Nautilus in 

 appearance and habits, but with a more flexible ' neck ' or stalk connecting 

 the anterior combination of head and foot (or Kopffuss) with the visceral 

 or mantle sac behind ; and he associated the origin of Gastropods with 

 a change of habits from swimming to creeping. The Nautiloid position 

 of the shell with the coil forwards and the aperture behind is regarded as 

 convenient for swimming (Prof. Naef does not note that this is only true 

 of backward swimming !), but is assumed to have been incompatible with 

 creeping, owing to pressure of the coil on the animal's head and neck. 

 A ' correction ' of these arrangements was therefore needed, which has 



