THE HISTORY 



OF THE 



CLASSIFICATIONS OF ZOOPHYTES. 



The existence of a polypiclom is not essential to a polype; 

 nor does it exercise, when present, that great influence over 

 the organization of its architects and tenants which might 

 have heen anticipated. The animal of the madreporous 

 Caryophyllsea does not essentially difl:er from the naked 

 Actinia ; and the gelatinous Hydra is a true representative 

 of the tenant of the sheathed Sertulariadse and Tubulariae. 

 No ascidian polype (Polyzoa), however, is ever found de- 

 tached, and without a polypidom ; and it is the same with 

 all our native Astroida. 



In reference to their composition, Polypidoms may be 

 divided into 1. the stony or calcareous, 2. the membrano- 

 calcareous, and 3. the horny and flexible ; but the line which 

 separates these divisions is often as uncertain and debatable 

 as that which is traced between the sister kingdoms. All 

 are composed of the same materials, viz. lime, and a gela- 

 tinous or membranaceous substance ; and their peculiar cha- 

 racters depend on the dilFerent proportions in which the 

 materials are mixed. The calcareous, which are hard and 

 inflexible, and, when dry, assume a white colour, consist 

 principally of carbonate of lime, with a small quantity of 

 the phosphate of the same earth, and the gelatinous matter 

 which cements them into one coherent mass, is in sparing 

 proportion : that proportion is so greatly increased in the 

 polypidoms of the second section, that when the earthy in- 

 gredients have been removed by the action of diluted acids, 

 the structure retains its original form, and is, in fact, reduced 

 to the condition of the polypidoms of the third section, which 



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