14 TOURNAL OF CONCHOLOGY, VOL. 9, NO. I, JAN., 1898. 



enemies. But a return to a condition of bilateral symmetry would be 

 as important for these swimming Gastropods as it is for the crawling 

 Opisthobranchs, and we do actually find in the anatomy of the different 

 genera of the Pteropods a series of stages in the gradual unwinding of 

 the body which this theory assumes. 



The story of the relations of the Pteropods as related by Boas 

 and Pelseneer is one that has found a place in every recent edition of 

 our hand-books of mollusca, so that it is not necessary for me to re- 

 hearse it now. I would simply point out to you that it is one which, 

 though differing in some important particulars, runs remarkably parallel 

 with the story of the untwisting of the Opisthobranch body. 



The last group of the Euthyneura which calls for comment is 

 that of the Pulmonata. There is very good reason for believing that 

 these animals have been derived independently of the other Euthy- 

 neura from a Streptoneurous ancestry. 



An operculum is retained in Amphibola through life, and is found 

 in the larval stages of Auricula and other genera. In Auricula, too, 

 we find a long visceral commissure, very unlike the very short com- 

 missure we find in Helix and Lim/icea. Moreover, in Auricula and 

 in Chilina we find a distinct asymmetry in this commissure similar to 

 that already referred to in Scaphander, which points most certainly to 

 a streptoneurous ancestry. It is probable that in taking to a terres- 

 trial habit, feeding on decaying vegetable matter in the marine 

 swamps, Auricula has escaped from many enemies that preyed upon 

 its ancestors, and for this reason has been able to dispense with the 

 protection afforded by an operculum. 



But the work required to drag the body and shell of an animal 

 through the air must be much greater than that required in sea-water, 

 and would in most cases need a greater exposure of foot and body 

 during locomotion. I do not know that any researches that have 

 been made comparing the amount of the exposure of the body of Pul- 

 monates during locomotion with that of the marine Streptoneura, but 

 I think it would probably be found if the matter were inquired into 

 that the former do expose their bodies more than the latter. If this 

 be actually the case, or even if it were true only of the ancestral Pul- 

 monates, it would be quite sufficient to account for the return of these 

 animals to a euthyneurous condition. It would mean, of course, the 

 rearrangement of the primitively asymmetrical organs into a secondary 

 symmetrical system, and this would be accompanied, as in the Opis- 

 thobranchs and Pteropods, by an untwisting of the visceral nerve loop. 



If it be not true, i.e., if the Pulmonata as a group do not expose 

 their bodies more than the others, then it must be confessed it is 

 difficult to account for their euthyneury. 



