41 



NOTES ON THE NERVOUS SYSTEM OF THE PELECYPODA. 

 By R. H. BuKNE, B.A. 



Read Uth December, 1903. 



In Nature'^ for October 29th is a letter from Mr. Latter, of 

 Charterhouse School, in which attention is called to the presence in 

 an individual specimen of Anodonta of a ganglionic enlargement 

 situated on the left cerebro-visceral connective in front of the peri- 

 cardium. Such abnormal ganglia have apparently been noted before, 

 and are of considerable interest in so far as they assist in the determi- 

 nation of the homologies of the different masses of ganglionic matter 

 that compose the central nervous system of the Pelecypods. 



In the case before us Mr. Latter, taking for granted the soundness 

 of the generally accepted view that in the cerebro-visceral system of 

 Pelecypods are to be found representatives of the cerebral and pleural 

 ganglia of the Gastropod in a degenerate condition, speaks of this 

 ganglion as the "pleural," and infers therefore that the Pelecypod 

 cerebral ganglion is cerebral pure and simple, strictly comparable to 

 the cerebral ganglion of the Cxastropod, and not, as is now most 

 frequently taught, a fusion of originally separate cerebral and pleural 

 ganglia. 



Assuming that any direct comparison between the individual nerve 

 centres of the cerebro-visceral system of Pelecypods and Gastropods 

 is justifiable, there seems little reason to criticise Mr. Latter's inter- 

 pretation of this abnormal ganglion, but as, for my part, I very much 

 doubt whether we have any right to such an assumption, I am glad to 

 take this opportunity to emphasise views lately put forward on this 

 subject by Dr. Gilman Drew,^ and to add some details in corroboration 

 of such views. 



The point of view, the objections to which I wish to urge, was 

 propounded by Pelseneer, and so far as concerns the Pelecypods is 

 briefly stated in the Comptes Rendus for 1890,^ and at greater length 

 in the Archives de Biol, fur the following year.* In these papers he 

 points to the presence in certain members of the Protobranchs of two 

 serially disposed ganglia in place of the simple cerebral ganglion 

 normal to the Pelecypoda, and also to separate connectives passing 

 from these two enlargements to the pedal ganglia, as evidence of 

 the possession by the Pelecypod stock of a nervous system of the 

 Gastropod type, characterised by independent cerebral and pleural 

 ganglia each united by a connective to the pedal ganglia. 



> Nature, vol. Ixviii (1903), p. 623. 



^ Drew, "The Life-History oi JVticula delphinodonta" : Quart. Joui'n. Micro. Sci., 

 vol. xliv (1901), p. 372. 



* Pelseneer, ' ' Sur I'ideutite de composition du systeme nerveux central des Pelecypodes 



et des autres Mollusques" : C.R. Ac. Sci., t. cxi, p. 245. 



* Pelseneer, " Contribution a I'etude des Lamellibranches " : Arch. Biol., t. xi. 



