THE DART SAC 



143 



Fig. 54. — Darts of British land snails : 

 A, Hyalinia e.rcavata Bean ; B, Helix 

 hortensis Miill. ; C, Helix asjicrsa 

 Mull. (After Aslifonl.) 



cutting purposes, Ixit simply to brace the stem. The dart is 

 contained in a dart sac, which is attached as a sort of pocket 

 to the vagina, at no great dis- 

 tance from its orifice. There are 

 four difterent 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 aspcrsa the 

 dart is about yV in. in length, 

 and -^r in. in breadth at its base 

 (see Fig. 54). 



It appears most probable that 

 the dart is employed as an ad- 

 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 dming the act 

 of copulation. It may be regarded, then, as an organ whose 

 punctures induce excitement preparatory to sexual union. It 

 only occurs in well-grown specimens. AVhen 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 Helicidac, a certain number 

 of exceptions Ijeing known which border on Helix. Hyalinia 

 nitidci, and cxeavata 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 Tchenno- 

 2'>horus. About one-third of the British Helices are destitute of 

 the dart.^ H. rufcscens 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 relati"\'e 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.- This theory, however, does not seem consistent with the 

 whole circumstances of the occurrence, position, and present use 

 of the darts. 



1 C. Ashford, Jo7(ru. of Conch, iii. p. 239, iv. pp. G9, lOS. 

 - W. E. Collinge, Zooloyist, 1S90, p. 276. 



