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THE MOLLUSCA 
J. INTRODUCTION. 
FORMERLY a number of very diverse zoological forms, such as 
Brachiopoda, 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 Vertebrata. 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 
