4i2 NATURAL SCIENCE. June, 



number of measurements of the shells to ascertain if there was any 

 constant difference between male and female. The male shells 

 average larger than the females, but there is an astonishing amount 

 of variation in the proportional dimensions of the shells of both sexes, 

 into which, however, I need not now enter. The essential differences 

 between the male and female shells will be seen clearly in the 

 accompanying figures (Fig. i), which are pen-and-ink tracings from 

 photographs kindly taken for me by Mr. Parkinson. In most adults 

 the shell can be readily identified as belonging to male or female, but 

 often the identification is very difficult, and in young shells impossible. 



The shell also varies to a great degree in thickness, being some- 

 times so thin that the slightest touch is almost sufficient to break it 

 at the free edge, and in other cases as thick as cardboard. 



6. Significance of the so-called post-anal or supra-anal papilla. — 

 This curious structure, which immediately strikes the eye on drawing 

 back the mantle, lies in the wall of the mantle across the middle line, 

 immediately behind 7 the region of the renal sacs and, in the female, 

 between that region and the nidamental gland. 8 It is identical in the 

 two sexes. Hitherto this has been of unknown insignificance, but I 

 have no hesitation in suggesting that it represents a pair of osphradia 

 or branchial sense organs, corresponding metamerically with the pair 

 described by Lankester and Bourne, between the bases of the gill 

 plumes. This conclusion rests at present on two main grounds, 

 namely, (i) Variation and (2) Relation to visceral nerves. 



As to the innervation, I will say at once that it is very difficult to 

 see the actual nerves or nerve-fibres (because the nerves are often not 

 compact trunks, but broken up into loose strands) which pass into the 

 osphradia ; but the anatomical relations of the visceral nerves to the 

 osphradia, which, I think, have never been fully described, are such 

 as to leave no doubt as to the source from which the osphradia derive 

 their innervation. 



In the first place, one readily sees in fresh specimens that we 

 have not to do with one papilla, but with a pair of papillae, which are 

 more or less deeply bifid or cleft at their free projecting extremities, 

 and so approximated to one another in the middle line as generally to 

 be fused together, but always with a deeper or shallower groove 

 between them. In two examples, I have observed that the two 

 papillae were absolutely independent of one another, and separated by 

 an interval (in one case, which I measured) of two and a half 

 millimetres. (Cf. Fig. 2.) 



Secondly, there are two main visceral nerves on each side, which 

 run side by side in the wall of the vena cava immediately below the 

 skin. On arriving at the region of the renal sacs, the two nerves 

 separate, one of them, the outer and larger, proceeding to the branchiae, 



7 That is when regarded with the mantle drawn back. 



K In the fresh condition the nidamental gland is of a bright yellow colour. 



