42 LAND AND FRESH-WATER SHELLS. 



The dart varies in shape in different species. It is to be found 

 only in the Helices * and in the slugs Agriolimax maculatus and 

 Tebennophorus. Mr. Ashford has made the admission that the 

 dart, in his thinking, is not thrust out of the body ; but he is 

 wrong, for it is forcibly ejected from one snail to the other before 

 they engage in coitus, and it is not a rare thing, when doing 

 field work, to pick up one of them from out of the grass. He 

 based his statement on the fact that when dissecting a dart out 

 of its sac it is always to be found connected at its base to the 

 posterior end of the lumen of that organ ; the mistake is here, 

 since the dart is secreted continuously by cells in the posterior 

 end of the sac, and this causes it to simulate an appearance as 

 if attached. The function which this dart serves in the economy 

 of the snail has not, as. yet, been thoroughly investigated. My 

 friend, Professor J. Bland Sutton, suggested to me that it is an 

 organ of cuticular irritation, and I find that Simroth holds the 

 same view ; but I cannot see any good reason to make this 

 hypothesis at all stable, for, if so, why is it not a constant feature 

 in the anatomy of the majority of snails? To me, the most 

 probable and the most feasible view, as yet, has been advanced by 

 another friend of mine, Mr. W. E. Collinge of Leeds. He believes 

 that in it we see the vestige of a structure which was once 

 constant in all snails, and which was used in former times as a 

 weapon of defence. Mr. Collinge is still collecting together facts 

 to support his view, and we shall all await the result with interest. 

 In H. hispida, H. rufescens, H. plebeia, and H. villosa there are 

 two dart-sacs, each containing a spiculum amoris. The Gas- 

 tropoda, with the exception, perhaps, of Limax lavis, are, as is 

 seen from the foregoing description, monoecious (Gr., monos, 

 single ; oikos, house) or hermaphrodite (Gr., Hermes, Mercury ; 

 Aphrodite^ Venus) animals. The generative act consists essentially 

 in an exchange of spermatophores, which are said, in H. pomatia, 

 to resolve in about ten days after the receival by the animal. 

 The spermatozoa set free by this resolution fertilise the ova in 

 the oviduct as they are descending from the ovotestis. By the 

 land-snails the eggs are laid singly in the earth or under stones 

 and logs of wood ; by the pond-snails they are laid in an 

 * It is rare in the American Helices ; so are the finger-shaped glands. 



