870 SUMMAKY OF CUREENT RESEARCHES RELATING TO 



Put shortly, the author's views are that the operculum of Gastro- 

 pods is not the homologue, either of the second valve of the shell, as 

 was thought by Gray, or the byssus of the Lamellibranchiata ; it is a 

 special production. The operculum is a calcified or horny production 

 of the epithelium ; the byssus that of a special gland. The pedal 

 gland is, on the other hand, the homologue of the byssus-gland. 



The author enters in great detail into the account of the structure 

 of the operculum and the foot, in a number of selected types of 

 Gastropods. 



Latent Period in the Muscles of Helix.* — H. de Varigny here 

 confines himself to an account of his observations on Helix pomatia, 

 ■which he has studied by the aid of induced currents, applied to the 

 two ends of the muscle of the foot. The period of latent excitation 

 is remarkably long, and varies from • 1 to 0-6 of a second ; the 

 period of contraction lasts much longer than in Vertebrates, and that 

 of relaxation may extend over several minutes. 



The extreme instability of equilibrium in the muscle is the next 

 point to be noted ; in other words, after removing a foot from the 

 rest of the body of the snail one may have to wait two or three 

 hours before it ceases to be excited by any external agent ; a truly 

 stable state is only reached when the muscle is quite or nearly dead. 

 It is almost impossible to experiment twice successively on the same 

 muscle in the same state ; and it is quite impossible to experiment on 

 one that has reached its maximum of extension ; the experiments, 

 therefore, that were made on the latent period of the mascle of Helix 

 were made with one that was more or less contracted. 



Affinities of Onchidia.f — E. Bergh, after discussing the views as 

 to the affinities of the OncMdia which have been held by preceding 

 writers, protests against the doctrine that they are nudibranchiate 

 molluscs, and claims them as decidedly pulmonate ; their nervous 

 system does not differ essentially from that of the Pulmonata ; it only 

 differs in having the lowermost part more condensed and reduced ; 

 the ophthalmophores are like those of the stylommatophorous Pul- 

 monata, and the pedal glands have very similar relations, as has too 

 the digestive system. It is true that the OncMdia are " opistho- 

 branchiate," but so are Arion and Limax ; in this group, at any rate, 

 the position of the heart has no systematic significance. The kidney 

 is very like that of the Pulmonata, and the difference between the 

 sizes of the lung-cavity is to be explained as due to the largely 

 cutaneous mode of respiration in the OncMdia, The most striking 

 proofs of relationship are to be found in the structure of the genera- 

 tive system ; the seminal duct has a position in the lateral wall of 

 the body, such as has never yet been demonstrated in any Nudi- 

 branch, but only in the Pulmonata. The OncMdia are Pulmonata 

 vphich have adapted themselves to an amphibiotic or marine mode of 

 life. 



* Comptes Kendus, xcix. (1884) pp. 334-7. 



t Morph. Jahrbuch, x. (1884) pp. 172-81. Cf. Ann. and Mag. Nat. Hist., 

 xiv. (1884) pp. 259-G6. 



