76 ASHFORD : ON THE DARTS OF BRITISH HELICID^. 



Gaster." Further observations are necessary but they are 

 difficult to make. In the course of above 500 dissections I have 

 met with about 20 decidedly immature darts. These have all 

 been wanting in annulus, expansion of the base, and (more or 

 less) in blades. From an examination of these few cases, thus 

 much seems to me evident: that the dart is not built up like a 

 factory chimney by successive accretions above: that the shaft 

 first appears as an excessively short and slender needle, attached 

 to the very apex of the tubercle: that it increases simultaneously 

 in length and breadth : that the blades are next formed from 

 the shaft outwards, by the addition of crystals along their edges 

 (evident from the jagged edge seen in a partially completed 

 blade): that the upper part of the blades, that namely nearest 

 the point, is completed before the lower portion : and, finally, 

 that the conical base and the annulus, by whatever part secreted, 

 are of later growth. I should add however, that I have met 

 with immature darts of only a few species. It is an interesting 

 fact, that in the earlier stages of growth it is difficult, if not 

 impossible, to determine the species from inspection of the 

 isolated dart, at least in those cases in which the shaft is quite 

 or nearly straight. To the question: "Where resides the 

 secretive power ? " no conclusive answer has been given. 

 Moquin-Tandon, if I understand him rightly, locates the 

 function in the tubercle alone. Several writers have adopted 

 Cuvier's suggestion, that the walls of the sac are concerned in 

 the formation of the blades. Leydig considers that the annulus 

 is formed by the tubercle, and the blades by the interior coating 

 of the sac. All these leave the shaft unaccounted for. 



Use. After much speculation upon this subject fifty or 

 more years ago, when Imagination frequently acted as Reason's 

 substitute, naturaUsts have settled down in the opinion that the 

 dart is simply an organ to induce by its puncture excitement 

 preparatory to pairing. Prof Semper, whose opinion is always 

 worthy of deep consideration, combats this view in the work 

 already quoted, and grounds his objection on the slenderness 



J.C, iv., July, 1883. 



