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CHAPTER X. 
MOLLUSCOIDA.—POLYZ DA. 
Tue Bryozoa, or Polyzoa—as British naturalists for good reasons 
prefer to call them—form the boundary-line which divides tne 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 Moiluscoida 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 Brachtopoda 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 little 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 
