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MOLLUSCS. 



MoUusca. 



Branch Glossophora 

 (Syn. Odontophora). 



Branch Lipocephala (Syn. Acephala). 



Class Cephalopoda — Nautilus and cuttle- 

 fishes. 



Class Lamellibranchiata (Syn. Pelecy- 

 poda, Conchifera or Bivalves (cockle 

 and mussel, clam and oyster). 



{Asynimetrical Anisopleura, 

 snails, whelks, etc. 

 More primitive, bilater- 

 ally symmetrical forms 

 — Isopleura, e.g.^ Chi- 

 ton. 



Unknown Ancestors. 



N.B. — The class Gasteropoda is somewhat heterogeneous, especially if it be made 

 to include the pelagic Pteropods. In the mean time, I have left out the small class 

 of Scaphopoda, but it may be inserted between Gasteropods and Cephalopods. 



General Notes on Molluscs. — The three great classes of 

 Molluscs — bivalves, snails, and cuttles — differ so much, that 

 it would be wearisome to discuss them together. A few 

 general notes, however, may find place here. 



As the word mollusca is but a Latinised form of the Greek 

 malakia, which means soft, it is necessary to explain that 

 Aristotle applied the title to the practically shell-less cuttle- 

 fishes, and that Linnsus used it to include Zoophytes, Anne- 

 lids, Echinoderms, and what we call naked molluscs. Our 

 shelled molluscs were called Testacea. Gradually, however, 

 the word " mollusc " prevailed, and of course oysters and 

 whelks are soft enough apart from their shells. The 

 technical history of " mollusca " well illustrates the progress 

 of classification, for within the last sixty years Cuvier's 

 Cirrhopoda (barnacles and acorn-shells) have been referred 

 to their proper place among Crustaceans, his Nuda (Tuni- 

 cates) have been raised to the rank of incipient Vertebrates, 

 the lamp-shells or Brachiopoda have been dismissed from 

 among Molluscs, and thrown along with Polyzoa into the 

 company of diverse " worms," or conveniently labelled " in- 

 cert» sedis." These changes have made the Mollusca into 

 a well-defined series. 



The shell is a very characteristic moUuscan structure, and 

 till we understand it better we must confess ourselves very 

 ignorant of the nature of Molluscs. But in spite of all the 

 years of conchology, we cannot answer the fundamental 

 questions about shell-making. Mollusc shells are very 



