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CHAPTER XXI. 

 OF THE CLASS OF POLYZOA. 



1070. THE animals forming the last class of the Mollusca pre- 

 sent such a close resemblance to the true Polypes in many par- 

 ticulars, that they were long regarded as forming an Order of 

 that class of Radiated animals, and it is only of late years that the 

 researches of Naturalists have shown that they really belong to 

 the Molluscous sub-kingdom, presenting considerable analogies to 

 the Tunicata and Palliobranchiata. They are always compound 

 animals, whence the name applied to the class,* but they some- 

 times, present themselves in an almost isolated form, with their cells 

 arising separately from a creeping stalk. In other instances 

 they are closely aggregated, as in the common Flustra, where 

 they are spread out into an expanded surface ; but they never 

 seem to exhibit the same degree of structural connection as that 

 which exists in the true Polypes. In a large proportion of in- 

 stances, the cells are of a delicate horny structure ; and in those 

 which are calcareous, the animal membrane remains as a much 

 more definite tissue, when the stony matter has been removed 

 by an acid, than in lithophyte corals. The wall of the cell is dis- 

 tinctly continuous with the membrane which closes the mouth 

 of it ; as this is with the external integument of the animal itself. 

 Moreover, the mouth of the cell generally possesses some flexi- 

 bility, and is drawn inwards, so as to form a kind of operculum 

 or lid to the cavity, when the animal retracts itself within. The 

 cells undergo remarkable modifications in form at different periods 

 of age, even in the calcareous polypidoms. The polypidoms or 

 polyzoaries of this group never attain any considerable size ; and 



* They are also called BRYOZOA by many writers, but the name here 

 adopted has the right of priority. 



