ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 415 



stracum, effect by their contraction the apposition of the soft shell 

 margins, and a consequent perfect closure of the shell. 



Opening of the Shell of Mussels.* — Dr. J. Pawlow has investi- 

 gated the mechanism of the opening of the shell in Anodonta cygnaea. 

 There is a nerve-ganglion 6-8 mm. in front of the anterior adductor 

 which gives off several branches. Some of them go to the ganglion on the 

 ventral surface of the posterior adductor. Observations were made by 

 clamping one valve of the shell to a firm board and connecting the 

 other by a silk thread with the short arm of a lever, the longer arm 

 of which works on a slowly rotating drum. An uninjured mussel 

 makes spontaneous movements, the valves being slowly opened a 

 little and closed again more quickly. After separation or irritation 

 of its proper ganglion, each muscle can be studied separately. 



The author sums up his conclusions as follows : — " Two classes of 

 nerve-fibres supply the adductor muscles, — (a) motor causing con- 

 traction, and (b) inhibitory interrupting the contraction and effecting 

 relaxation. The motor nerves of each muscle spring from the nearest 

 ganglion ; but all the inhibitory fibres originate in the anterior 

 ganglia. The latter pass to the anterior adductor by the short nerve- 

 branches which pass to it from the anterior ganglia. They reach 

 the posterior muscle through the connectives. The posterior ganglion 

 thus functions as motor centre for the posterior adductor ; and the 

 anterior ganglia act similarly on the anterior adductor. The motor 

 cells of the ganglia on either side may be stimulated to activity, 

 either by peripheral nerve-fibres (of the mantle or gills), or by 

 certain fibres of the connectives. The anterior ganglia are able to 

 produce relaxation in either anterior or posterior adductors." 



Resting position of Oysters.j — Mr. S. Saunders considers that 

 Mr. J. T. Cuningham| is right in stating that oysters are usually 

 found with the left (convex) valve uppermost ; and that Prof. Huxley 

 is also correct in stating that young oysters are invariably attached 

 by this left valve. The discrepancy is explained by the fact that the 

 oyster, during its first or second year, becomes detached either by 

 the dredger or by natural means. The oyster, falling on its convex 

 valve, will get turned over by the motion of the water, and then 

 remain on its flat valve, being now free from disturbance by the water. 

 Moreover, if it remains attached by the convex valve, the mud, &c., 

 would tend to remain in its concavity and thus injure the soft parts, 

 whereas if the flat valve be undermost the motion of the water can 

 more easily sweep away any foreign matter from the shell. 



'Challenger' Lamellibranchiata.§— Mr. E. A. Smith confines 

 himself chiefly to descriptions of the shells of the Lamellibranchs 

 collected during the voyage of H.M.S. ' Challenger.' He urges the 

 priority of the term Pelecypoda given to the group by Goldfuss. 

 Only about 500 species were obtained, and the greater number came 

 from shallow waters; only one new genus is described. Lamelli- 



* Pfluger's Archiv, xxxvii. (1885) pp. 6-31 (1 pi.). 



t Zoologist, X. (1886) pp. 114-5. J See tliis Journal, ante, p. 52. 



§ Keports of the Voyage of H.M.S. ' Challenger,' xiii. (1885) 341 pp. (25 pis.). 



