THE MORPHOLOGY OF THE MOLLUSCA. 



^ > H E recent publication of a number of new manuals and 

 monographs dealing with the Mollusca offers a favour- 

 able opportunity for a review of our knowledge of this 

 great phylum of the animal kingdom. It is not fifteen 

 years since Professor Lankester's classical article on Mollusca 

 was published in the Encyclopedia Britannica, yet the con- 

 tributions to Molluscan morphology since that date have 

 been not only numerous, but in many cases of prime im- 

 portance. 



The older method of inquiry, that of the comparison of 

 types more or less arbitrarily selected from different groups, 

 has been succeeded by investigations more directly in- 

 fluenced by the idea of evolution. The comparison of types 

 has been replaced by the study of groups. The founda- 

 tions of the morphological edifice were laid upon the former 

 method ; the superstructure and details are the result of 

 the latter. Homologies having been to a large extent 

 determined, we now seek phylogenies. It happens also 

 from time to time that the detailed study of a group with 

 the object of reconstructing the phylogeny of its members 

 leads occasionally to the discovery that homologies based 

 on the simple method of anatomical comparison turn out 

 to be nothing more than analogies — recurrent examples of 

 similar modifications. 



One result of these phylogenetic inquiries has been the 

 concentration of particular attention upon forms which are 

 presumably the most primitive in each group ; and great 

 advances have thus been made in our knowledge. Kow- 

 alewsky and Marion, Pruvot, Wiren, and Thiele have 

 enormously extended our acquaintance with the Apla- 

 cophorous Isopleura ; primitive Prosobranchs (Docoglossa 

 and Rhipidoglossa) have been thoroughly investigated by 

 Haller and Boutan ; Bouvier has thrown new light upon the 

 Opisthobranchia by his researches on Actceon ; Boas and 

 Pelseneer have revolutionised our ideas of the Pteropoda 



