622 NOTES ON PROSOBRANCHIATA, 



forming part of a general similarity of organisation. Returning 

 to the osphradium, Pelseneer (16) has said : — " II constitute alors 

 (dans les Tenioglosses les plus archaique, exemple Paludina^ 

 Littoriyia, Cyclostoma, Vermetics, etc.), un bourrelet epithelial 

 filiforme, sur un nerf ou sur un ganglion." While not contesting 

 that this type of osphradium is archaic, I would point out that 

 in some instances it may be a degenerate form. For instance, 

 concomitant with a habitat such as that affected by Littorina, 

 Tectarius, etc., there would necessaril}^ be a disuse of this organ, 

 a condition favourable to degeneration. The presence of an 

 osphradium in a mollusc endowed with a pulmonary cavity would 

 seem to mark such as an evolving Pulmoiiate rather than an 

 archaic Tsenioglossan. 



Although somewhat foreign to the present paper, a short 

 speculation on the use of the " glande pedieuse " of Vermetus may 

 perhaps be allowable here. In the words of Lacaze Duthiers (10), 

 the osphradium is here " reduit a un filet." The species of this 

 genus are almost always covered by water, and, being fixed 

 permanently in one position, it must be a vital necessity to test 

 the quality of the water they inhale; and yet the osphradium, 

 although strongly innervated, is very simple. It has occurred 

 to the writer that it may be that the so-called pedal gland is a 

 highly specialised olfactory organ; its free communication with 

 the water would seem to support this view. It may be worthy 

 of note that the analogue of such a pedal olfactory organ is to 

 be found among the Pulmonata. That it is not a mucous gland 

 seems certain from Lacaze Duthier's paragraph (10,265): — "Une 

 particularite bien digne d'interet s'est presentee. La glande placee 

 dans la cavite du corps, qui s'ouvre entre le pied et la tete est, 

 tres probablement I'intermediaire entre la cavite generale du 

 corps et I'exterieur. Si done il n'a pas ete possible de decouvrir 

 ailleurs I'orifice exterieur de la circulation, on pourrait le con- 

 siderer comme existant a la fa,ce inferieure de la glande." This 

 explanation of its function seems untenable, since Vermetus is 

 not capable of much expansion, and one would not, therefore, look 

 to find a highly specialised aqueo-vascular orifice developed. 



