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CHAPTER X. 



MOLLUSCOIDA. — POLYZOA. 



The Br>'ozoa, or Polyzoa — as British naturalists for good reasons 

 prefer to call them — form the boundary-line which divides the humble 

 mollusc from the humbler zoophytes. In consequence of this inter- 

 mediate organisation, these creatures were long considered as polyps; 

 but De Blainville, Milne-Edwards, Ehrenberg, and Vaughan Thomp- 

 son, almost simultaneously began to separate them from the molluscs, 

 and form them into a separate group. Subsequent naturalists, while 

 considering the Molluscoida as truly and wholly molluscous, admit 

 that the distinction proposed by Milne-Edwards, 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 also place the Brachiopoda in 

 the group Molluscoida. 



Marine plants are sometimes observed to be quite covered with 

 velvety parasites, which might at a first glance be mistaken for a sea 

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

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

 neighbour. 



These litde creatures are thus entirely distinct. Each cell, is 

 formed by the integument, which has been encrusted by calcareous 

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

 This kind of covering protects the animal from the attacks of its 

 enemies. This mode of retreat at the bottom of a protecting shell 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 Polyzoa has long 

 been mistaken in some forms at least for Zoantharian corals. 



_ Each animal has its own opening, and is furnished with a dentate, 

 spinous enclosure, or protected by an operculum or lid ; they present 

 themselves under every variety of form, sometimes as an assemblage 



