ASHFORD : ON THE DARTS OF BRITISH HELICID^. 7 1 



to Semper, of transverse and longitudinal layers of muscle- 

 fibre. This coat is semi-transparent, slightly translucent or quite 

 opaque, and in colour pearly-white, grey, yellowish or light 

 purple, sometimes spotted externally with small scattered or 

 closely-set brown specks. The extremity is often thinner than 

 the rest, whereby the eversion of the sac would appear to be 

 rendered more easy. The aperture has usually two or more lip- 

 like prominences (pi. ii. fig. 4). Within, and more or less intimately 

 adherent to the outer, is another coat, thinner, less firm in 

 texture, generally of the same colour as the first, but in the case 

 of a few species densely charged with chocolate-black or violet- 

 brown pigment cells, so much so as to give a livid aspect to the 

 whole organ when viewed externally. This colouring, when it 

 exists, is intensified at the neck of the sac, and extends to other 

 connected organs (e.g., H. netnoralis). A lining membrane 

 secretes a lubricating fluid of a viscid character. At the fundus 

 of the inner cavity is a small sub-conical tubercle, first noticed 

 by Martin Lister and supposed to play an important part in the 

 formation of the dart. In If. aspersa this tubercle has a white 

 nucleus, which, on desiccation, yields an amorphous mineral 

 matter effervescing in hydrochloric acid. In H. netnoralis and 

 some others it shares the pigment of the inner coat, of which in 

 fact it is an offset. Whatever be the scope of the secretive 

 function of this little protuberance, it certainly serves as a basis 

 of attachment for the dart, whose point is thus directed towards 

 the aperture (fig. 4). When the sac is dissected from its position 

 of repose in the mantle cavity, it is frequently found — more 

 especially in the case of single simple sacs- — to be so acutely 

 bent at the neck, that for the moment a wonder arises as to how 

 the sharp-pointed weapon can possibly effect its exit, but it must 

 be remembered that when the animal is extended, the sac, as 

 has already been observed, is drawn forwards towards the right 

 tentacle, in which position the flexure referred to becomes 

 obhterated, and the axes of the sac and vestibule and the centre 

 of the external aperture assume a nearly direct line. Then by 



