210 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and spiral, as in Avion rufus. Its walls are formed by a layer of 

 mesodermal cells with a large nucleus, surrounded by a contractile 

 irregularly stellate protoplasm, the branching processes of which 

 unite with one another. Externally to this layer is a finely ciliated 

 ectoderm ; the contained cavity, which exhibits diastole and systole, 

 communicates with the body-cavity, and receives from it and returns 

 to it fluid. Shortly before the young slug leaves the egg the podocyst 

 is completely absorbed. In considering the function of this embryonic 

 organ, we have to note that it is in direct contact with the inner 

 surface of the shell, so that it occupies a very favourable position for 

 the exchange of the necessary gases between the blood and the sur- 

 rounding air ; on the other hand, it is in direct relation to the reserve 

 of material which is used up by the embryo. From a physiological 

 point of view, then, it seems to be comparable to the allantois of the 

 higher Vertebrata. 



There does not appear to be any good reason for thinking that 

 the prepallial swelling is a contractile sac, which acts antagonistically 

 to the podocyst ; all the mesodermal tissues alike distend when the 

 podocyst contracts, and it is only in consequence of the looser texture 

 of the swelling that its movements of dilatation and contraction are 

 more marked and more easily visible. 



Spicula Amoris of British Helices* — C. Ashford contributes a 

 comprehensive paper on the " darts " found in connection with the 

 reproductive apparatus in certain Helices. 



The dart is contained in a short ventricose pouch opening into the 

 lower part of the vaginal tube, a little above the common vestibule, 

 on the right side of the neck. There is usually one : if two are present, 

 the second sac is on the opposite side of the tube from the first. The 

 sac may be simple or bilobate. At the bottom of the cavity of the 

 sac is a conical papilla, which serves as a basis for the dart, which is 

 attached to it by its posterior end. The apparatus is a development 

 of adult life, and especially of pairing time, but this is indifferently 

 present or wanting in species otherwise closely allied. The dart 

 itself is a tubular shaft, of carbonate of lime, tapering to a solid, 

 transparent, sharp point, enlarging at or towards the base, where it 

 assumes the form of a subconical cup. The sides of the shaft are 

 sometimes furnished with blade-like longitudinal buttresses, which 

 serve to strengthen it. They are rapidly formed, may be secreted in 

 six days, and differ in form in different species. They are supj)osed 

 to serve the purpose of inducing, by puncture, the excitement pre- 

 paratory to pairing. They are too fragile to do more than prick the 

 tough skin of these molluscs, but sometimes penetrate the apertures 

 of the body, and are found within. A new weapon is formed after 

 the loss of the old one. It is best extracted for study by boiling the 

 sac in caustic potash. 



Anatomy of Pelta and Tylodina.f — A. Vayssiere gives an account 

 of these small and incompletely known molluscs. The presence of 



* Journ. of Conch., July 1883. Cf. Science, ii. (1883) p. 803. 

 t Ann. Sci. Nat. Zool., xv. (1883) Art. 1, 46 pp. (3 pis,). 



