12 JOURNAL OF CONCHOLOGY, VOL. 9, NO. I, JAN., 1 898. 



I do not propose in this address to discuss any particular theory 

 as to the origin of the twist in the evolution of the Gastropoda. 

 Those of you who are interested in this subject will find an 

 excellent summary of what appears to me to be the most reasonable 

 theory in Lang's Text-book of Comparative Anatomy (English edition, 

 vol. ii., p. 150). The summary that he gives is of importance to us : 

 "The formation of a spire-like shell which has been recognised as the 

 starting-point in the development of the asymmetry of reptant Gastro- 

 pods, was the only method by which complete protection of the whole 

 body could be attained, and must therefore be considered to have 

 been advantageous under the circumstances." 



The loss of the organs of one side of the body and the accom- 

 panying twist in the visceral hump and the visceral nerve chord, were 

 due to the gradual evolution of a form of shell which would most 

 efficiently protect the soft molluscan body. The need for protection 

 arose from the danger they were exposed to of falling a prey to car- 

 nivorous fish and other marine animals, for we find that all, or nearly 

 all, the Streptoneura protected by a hard shell closed by a strong 

 operculum are edible. They are serviceable as bait for fishes, and in 

 many cases are appetizing to the more critical palate of man himself. 

 The reason for the untwisting is not so easy to understand, but I 

 venture to put before you for consideration a view which appears to 

 me to have some truth in it, and may lead to some further and much- 

 needed investigations on the habits of the Opisthobranchs. 



It might be asked in the first place — Why should the Euthyneura 

 have become untwisted and have lost their shell if the streptoneurous 

 condition was such a valuable protection to their bodies from the 

 assaults of their enemies ? The answer to this question would be 

 that a heavy spiral shell must be a great encumbrance to the body 

 and impede the locomotion of the animal. It is an encumbrance not 

 only in its weight, but from the fact that to completely protect the 

 whole body the foot must be relatively small. It is, I believe, a well- 

 known fact to conchologists that Streptoneura, like the whelks, 

 winkles, muricidce, purpuras, and others, that can completely with- 

 draw their bodies into their shell, do not crawl fast nor wander far. 

 They are dependent upon food that is near at hand. 



Now, any increase in the size of the foot, bringing with it in- 

 creased powers of locomotion, would be an advantage to the species 

 in giving it a wider range, but would be a disadvantage to it in leaving 

 the body more exposed. The only other way in which this danger 

 could be met, would be by the adoption of some other method of 

 protection. If, for example, the foot developed glands that secreted 

 a poisonous or disgusting fluid which the enemies of Gastropods 



