THE MOLLUSC A 



I. INTRODUCTION. 



FORMERLY a number of very diverse zoological forms, such as 

 BrachiogQda, Tunicata, and even Cirrhipedia, were included among 

 the Mollusca. The last-named were the first to be dissociated from 

 the true Mollusca, after J. Vaughan Thompson had shown, by a 

 study of their larval development and metamorphoses, that they 

 were modified Crustacea. At a later date the Tunicates were 

 shown, also as a result of embryological research, to have affinities 

 with the Yertebrata. The Brachiopoda. both because of their 

 bivalve shell and the supposed resemblance of their superficial 

 anatomy to that of Lamellibranchia, retained a place in the 

 Molluscan phylum for a much longer period. H. Milne-Edwards, 

 it is true, united the Brachiopoda with the Polyzoa under the name 

 Molluscoidea, but he placed this group very close to the Mollusca, 

 and his views were very generally accepted by zoologists. It is 

 only twenty-five years since the phylum Mollusca was finally 

 purged of all alien elements, and limited, as in Professor Lankester's 

 "Mollusca" in the ninth edition of the Encyclopedia Britannica, 1883, 

 to such forms as are demonstrably related to one another. 



More recently H. von Jhering propounded the view that this 

 assemblage of animals, believed to be closely related to one another, 

 was in fact unnatural, heterogeneous, and polyphyletic. But his 

 hypothesis could not stand the test of criticism, and never ob- 

 tained the support of any reputable malacologist. Since his 

 time the unity of the Molluscan phylum has never been called into 

 question. 



Closely related as the different forms of the Mollusca are, 

 they exhibit a great variety in external aspect, chiefly because the 

 tegumentary layer, consisting of epithelium, connective tissue, and 

 muscle, is exceedingly plastic, and gives rise to outgrowths, 

 appendages and expansions of the most various kind. The 

 diversity of form is further increased by concrescence of the 

 various out-growths of the body, either with one another, or 

 with adjacent structures ; such concrescences being exhibited by 

 the mantle edges, the lobes and margins of the foot, the gills, and 

 other organs. 



But, however diverse the external configuration of the Molluscan 

 body, the internal organisation, at least in its main features and 

 in young forms, preserves a remarkable uniformity. The group 

 is homogeneous, sharply defined, and its members are easily 

 recognised. The Mollusca also afford a very good instance of the 

 progressive modification and evolution of organic structure. It 

 would be difficult to name another group of the animal kingdom 

 in which relationships can be more clearly determined and the 



