68 PKOCEEDINGS OF THE MALACOLOGICAL SOCIETY, 



this position established, than Pelseneer, also relying upon the study 

 of internal as well as external characters, builds up a no less forcible 

 argument ' for the retention of the genus among the Pulmonata. 



As the necessity for a fuller study of the Molluscan nervous system 

 comes here prominently forward, allow me to direct your attention to 

 two somewhat earlier investigations of the same category, by which one 

 of our most novel systems of classification has been brought under the 

 action of time. I refer to that based on Spengel's important work 

 upon the nervous system and sensory apparatus. Actceon, judged by 

 the structure of its shell and its persistent operculum, would appear 

 rightly placed among the lower Opisthobranchs. During a careful 

 anatomical investigation, Bouvier has made the important discovery^ 

 that it has a twist in its visceral nerve-loop. Plate follows in the 

 wake, with the discovery ^ that Chilina is also streptoneurous ; where- 

 fore, from the study of their nervous system alone, we might place 

 these two genera among the Prosobranchs. From what is known of 

 their general organization this would be an unwarrantable proceeding, 

 and the only alternative is the conclusion that we have here to deal 

 with an inherited streptoneury. If we are justified in this, the 

 argument that the cuthyneurous condition of the Opisthobrancliiates 

 and Pulmonates points to an orthoneurous ancestry must be held in 

 abeyance. 



Opisthobranchs and Pulmonates from Prosobranchs ! A revolu- 

 tionary deduction from a comparatively simple anatomical fact; but 

 from what we know of the nervous system as a guide to affinity and 

 inter-relationship among other great groups of animals, it is justified. 



We may here most fittingly record the observation of Dr. J. D. 

 Gilchrist* that although in the untwisting of the Gastropod body 

 consequent on the reduction and loss of the shell, the original bilateral 

 symmetry may be apparently resumed, the organs originally lost under 

 the mechanical effects of torsion do not reappear. I cannot dismiss 

 this paper without diverting your attention to Dr. Gilchrist's ingenious 

 proposal to simplify our conceptions and our classifications by dis- 

 criminating between Mono-, Di-, and Poly-placophora, expressing 

 the hope that in his new office at the Cape he will not forsake the 

 Mollusca. 



Passing to the Isopleura, we note the discovery by Haller^ of the 

 recurrence of auriculo-ventricular valves, there being four pairs in 

 Chiton GoodalU.^ Our member, Mr. R. H. Burne, has partly 

 confirmed Sailer's observations during the year. The occurrence of 

 recurrent symmetry of parts of the bodies of these Molluscs which 

 are popularly regarded as approximate to the worms, is of manifest 

 interest; but the facts appear to me to support the conclusion now 



1 Mem. cour. 4°, Acad. Sci. Belj^., torn, liii, No. 8. 



2 Comptes Eend. Auat. Sci. Paris, torn, cxvi, p. 68. 



3 Zool. Jahrb. Anat. Abtla., Bd. ix, p. 200. 

 * Proc. Roy. Soc. Edinb., vol. xx, p. 357. 

 5 Morph. Jahrb., Bd. xxi, p. 28. 



" Originally described as C. magnifciis. Cf. Zool. Anz., Bd. xix, p. 200. 



