THE DART SAC 1 43 



tapering to a fine point above, and enlarging gradually, more 



often somewhat abruptly, to the base.' The sides of the shaft 



are sometimes furnished with two or more blades ; these are 



apparently not for cutting purposes, but simply to brace the ste;n. 



The dart is contained in a dart sac, which is attached as a sort 



of pocket to the vagina, at no great distance from its orifice. 



There are four different forms of 



sac. It may be single or double, 



and each of these divisions may 



be bilobed, each lobe containing 



one dart at a time. In Helix as- 



persa the dart is about -f^ in. in 



length, and ^ in. in breadth at 



its base (see Fig. 54). a ts c 



-r, , T_ r 1 xT_ J. ^iGt. 54. — Darts of British land snails: 



It appears most probable that a, HyaUnia excavata Bean; B, Helix 

 the dart is employed as an ad- hortensis Miill.; C, Helix aspersa 

 ^ ^ ^, 1 ^ -o • J Miill. (After Ashford.) 



junct to the sexual act. Besides 



the fact of the position of the dart sac anatomically, we find 

 that the darts are extruded and become embedded in the flesh 

 just before or during the act of copulation. It may be regarded, 

 then, as an organ whose punctures induce excitement prepara- 

 tory to sexual union. It only occurs in well-grown specimens. 

 When once it begins to form, it grows very rapidly, perhaps 

 not more than a week being required for its entire formation. 



The dart is almost confined to Helicidae^ a certain number 

 of exceptions being known which border on Helix. HyaUnia 

 nitida and excavata are the only British species, not Helices^ 

 which are known to possess it. It has not been noticed to 

 occur in the slugs, except in the N. American genus Tehenno- 

 phorus. About one-third of the British Helices are destitute of 

 the dart.i H rufescens possesses a double bilobed sac, but only 

 two darts, which lie in the lower lobes. It does not use the 

 darts, and could not do so, from the relative sizes of dart and 

 sac ; it has often been watched when uniting, but the use of the 

 darts has never been observed. From this it has been inferred 

 that the darts are degenerate weapons of defence, and that they 

 were in fact at one time much stronger organs and more often 

 used.2 This theory, however, does not seem consistent with 



1 C. Ashford, Journ. of Conch, iii. p. 239, iv. pp. 69, 108. 



2 W. E. Collinge, Zoologist, 1890, p. 276, 



