38 THE VOYAGE OF H.M.S. CHALLENGER. 



specially directed ; the circulation, the entrance of water into the blood, pori aquiferi, 

 and the like, have mainly occupied attention. Thus it is that our notions of the mutual 

 relations of the different groups of Peleeypoda are still very imperfect. 



In order to reach a completer view of the class, it would be necessary to have 

 material from all parts of the world, and representative of all the principal types, and 

 then in such a museum to resume the labours of Poll and Deshayes with the light of 

 the phylogenetic conceptions which were unknown to them. 



But although there are great lacunas in our knowledge of the morphology of 

 Peleeypoda, I do not think that it is necessary to abstain from already seeking to 

 discover the mutual relations of the different groups, or, in other words, to sketch their 

 phylogenetic or natural classification. 



Indeed, however imperfect the result of any such endeavour may be, it cannot but 

 be of use in attracting discussion and criticism, and thereby suggesting new researches. 

 From this point of view one cannot help regretting that there are so few synthetic works 

 among the multitude of analytic or descriptive memoirs which see the light, and so few 

 ideas amid so many accumulated facts. For every new idea is a progress ; and even 

 when synthetic attempts or theoretical ideas are pushed to an extreme, as, for example, 

 in the great work of von Jhering on the nervous system of Molluscs, they do not by 

 any means remain barren of results, even if they only serve to incite to research and to 

 provoke discussion. 



But even von Jhering has not attempted to sketch the phylogenetic classification 

 of the Peleeypoda. The only attempt of the kind, and that from an exclusively con- 

 ch ological and palaeontological point of view, is due to Neumayr.'' 



While I share the opinion of Fischer ^ as to the limited value of the hinge in 

 classification, I cannot, on the other Iiand, ignore the work of Neumayr, or the results 

 of his conchological and palaeontological researches. For although Professor Gaudry 

 has described Palaeontology as " grandeur et misere," it must be allowed that as regards 

 MoUusca (except from some points of view in respect to Cephalopods), Palaeontology is 

 hardly anything but " misere." In spite of the perfect fossils that remain, nothing of 

 the real structure is revealed beyond certain peculiarities of the mantle. 



Therefore I limit myself here simply to the living Peleeypoda, and exclusively to 

 the study of their soft parts. In the following table, only the great groups are indi- 

 cated. Except in the case of a few important families, I have omitted the smaller 

 groups which gravitate round the larger. It must be noted, furthermore, that this 

 table is only a sketch, designed especially to indicate the successive modifications of the 

 gills, and to summarise the facts which have been discussed above. 



' Zur Morpholrgie des Bivalvcnsclilosses, Silziiiig.sb. d. k. k: Akad. Wlss. Wien, Bd. Ixxxviii. p. 413. 

 - Uue nouvelle Classification des Bivalves, Journ. de Coiichi/l., t. xxxii. p. 121. 



