yo ASHFORD : ON THE DARTS OF BRITISH HELICID^. 



mantle cavity when the animal is withdrawn into its shell, but 

 is dragged forward along the right side of the neck when the 

 snail is crawling. When two exist they generally occupy opposite 

 sides of the vagina. The mucous glands with their simple or 

 digitate coeca have their outlet near, but above, that of the 

 dart-sac. The function of these glands still remains in doubt, 

 but the fact that they seldom exist alone and are very rarely 

 absent when the dart-sac is present, is suggestive, and the two 

 organs should be studied together. The function of the sac is 

 three-fold— to secrete, preserve and protrude the dart. 



Form. The occurring British forms, though varying much 

 in proportion and contour, fall naturally into four groups, marked 

 by the respective characteristics of (i) One simple sac; (2) A 

 single bi-lobed sac; (3) Two simple tehferous sacs; (4) Two 

 sacs, each bi-lobed (fig. 3). In addition, we have one or two 

 species (e.g., II. obvoluta) with the organ in a rudimentary or 

 degenerated state. Such forms do not develope darts. It is 

 not to be expected, considering the small number of dart-bearing 

 species in this country, that these four groups should be 

 connected by fine gradations. Indeed, there are very few 

 passage- forms. The case would be otherwise were the compari- 

 son extended to foreign species. Thus, Moquin-Tandon feels 

 justified in stating, with regard to the 50 or 60 dart-bearing 

 species he recognises as occurring in France: "On trouve toutes 

 les nuances possibles entre les bourses simples et les bourses 

 bilobees..." Without taking the phrase "all possible gradations" 

 literally, there is ample evidence of a course of development 

 from a simple to a more complex organ. — The shape of the 

 dart-sac is remarkably persistent among the individuals of a 

 species. The only variation I have noticed is afforded by H. 

 virgata. In that species the extremity of the dart-sac is some- 

 times truncated or obtusely notched, and in that state may 

 perhaps be considered a link between groups i and 2. 



Structure. In the course of dissection there will be 

 observed, first a thick muscular outer coat, consisting, according 



J.C, iv., July, 1883. 



