Relationships with Other Phyla.—A great deal of discus sion has been devoted to the origin of the Mollusca. There are, however, two main and radically opposed views on this subject. The one emphasizes the resemblance between the trochophore larva of the Mollusca and the similar larva of the Annelida, and holds that the Mollusca are descended from the same ancestral form as the Annelida. The other theory (propounded by Lang and Thiele) stresses certain structural features of the adult as well as embryological data, and its authors would derive the Mollusca from the free-living Platyhelmia (Turbellaria). It is sufficient to say here that at present this question is unsettled, though the majority of zoologists would probably incline to the view that the Mollusca are more closely related to the Annelida than to any other phylum.
Historical.—The first systematic study of the Mollusca is due to Aristotle, who described various kinds of Cephalopoda, Gastro poda, etc., in his Historia Animalium and de Partibus Animalium and noted their habits with considerable accuracy. He made, how ever, a radical distinction between the naked forms and shelled forms which is no longer recognized.
The foundation of the modern study of the group may be attributed to Martin Lister, who in his Historiae conchyliorum (1685) gave anatomical and conchological descriptions of many Gastropoda and Lamellibranchia ; but the group was for a long time imperfectly understood and the "testaceous" Mollusca were still kept separate by Linnaeus from the "naked" form and asso ciated with different groups of invertebrates. Linnaeus's "Mol
lusca" (1758) were a heterogeneous assemblage and did not in clude the shell-bearing forms. The work of Cuvier (1799), how ever, who assigned to the Mollusca a position as one of the chief primary groups of the animal kingdom, did much to clarify con temporary knowledge of this group. Among the naturalists of that period Poli and delle Chiaje made important investigations into the anatomy of various molluscs. It should be pointed out that the study of the Mollusca has always been characterized by an in evitable duality of method. On the one hand, conchological studies upon the form and the sculpture of the shell have contributed to our knowledge of the specific and generic variety of the group as well as of the geographical distribution of its members. Of such studies may be mentioned the earlier treatises of Martini and Chemnitz, Reeve and Sowerby and the later studies of Adams, Gwynn Jeffreys, Alder and Hancock, Von Martens, E. A. Smith and H. Fischer. On the other hand, the recognition of the limits and character of the group and the elimination from it of mem bers of other phyla are due to anatomical and embryological re search of which the most distinguished exponents have been Vaughan Thompson, Kowalevski, Milne Edwards, Lacaze Du thiers, Kolliker, Spengel, Owen, Huxley, Lankester and Pelseneer.
(G. C. R.)