THE MORPHOLOGY OF THE MOLLUSC A, 39 



by their work upon Limacina among the Thecosomata, and 

 upon Dexiobranchcea and other types among the Gymnoso- 

 mata ; the morphology of the Pelecypoda has been further 

 elucidated by Pelseneer's observations upon Nucula and 

 other primitive forms, and important contributions to our 

 knowledge of the Cephalopoda were made during the past 

 year by Huxley and Pelseneer in the case of Spirilla, that 

 last survivor of the ancient types of Decapod Dibranchiates. 

 We doubt if any equivalent group of the animal kingdom, 

 except perhaps the Echinoderma, has been the subject of 

 such productive researches as the M ollusca during the period 

 under consideration ; and certainly the phylogenetic method 

 of inquiry has attained no greater triumphs than in the 

 hands of Bouvier, Haller, Pelseneer, and other inves- 

 tigators of the Gastropod and Lamellibranch series. 



In the present article I propose to deal more especially 

 with recent contributions to our knowledge of the Molluscan 

 nervous system, reserving a fuller consideration of other 

 questions for a later article. 



There is one writer, however, whose views must first of 

 all be dealt with, as on a great number of fundamental 

 points they are opposed to all current conceptions of 

 Molluscan morphology. These views merit some detailed 

 consideration, moreover, for they are based on propositions 

 which are not without a certain appearance of plausibility, 

 and may well serve as test-questions by which to examine 

 into the accuracv of the homologies which have been 

 generally admitted to exist between the different sections 

 of the Molluscan phylum. 



Thiele has published his views in a series of lengthy 

 papers, the references to which will be found in the biblio- 

 graphy (23, 24, 25). He regards the Mollusca and Anne- 

 lida as direct descendants of Polyclad Turbellarians, and 

 his identifications of homologous organs in the different 

 Molluscan groups are determined, not by a direct comparison 

 of the organisation of these types one with another, but 

 by independent comparisons of the organisation of the 

 different Molluscan types with that of sucker-bearing 

 Polyclads. The group Mollusca is thus made to lose its 



