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CHAPTEK X. 



MOLLUSCOIDA. 



THE Bryozoaires, or Polyzoa, as British naturalists prefer to call them, 

 form the boundary-line which divides the humble mollusc from the 

 humbler zoophytes. In consequence of this intermediate organi- 

 zation, these creatures were long considered as polyps ; but De Blain- 

 ville, Milne Edwards, and Ehrenberg, almost simultaneously began to 

 separate them from the molluscs, and form them into a separate group. 

 Subsequent naturalists, while considering the Mollusco'ida as truly and 

 wholly molluscous, admit that the distinction proposed by the French 

 naturalists is most important, and should be retained as a primary 

 subdivision, confining it to those molluscs which have the neural 

 region comparatively little developed, and the nervous system reduced 

 to a single or at most a pair of ganglia, and the mouth surrounded, 

 by a more or less perfect circle of tentacles : an arrangement which, 

 would include the BracMopoda with the Polyzoa. 



Marine plants are sometimes observed to be quite covered with a 

 velvety parasitic matter, which may at a first glance be mistaken for a 

 moss. This, however, is simply an aggregation of animalcules, each 

 of which has its separate cell, which is placed quite contiguous to its 

 neighbour. 



These little creatures are thus entirely distinct. Each cell is 

 formed by the skin, which has been encrusted by calcareous salts, or 

 other organic matter, hardened after the manner of a horn. This 

 kind of shell protects the animal from the attacks of its enemies. 

 This mode of retreat at the bottom of a protecting shelter is very 

 frequently adopted in the whole series of molluscs. The oyster shuts 

 itself up by closing its valves, and the snail retires into its shell. This 

 assemblage of small cells presented by the Bryozoaires has long been 



