612 THE WONDERS OF GEOLOGY. Lect. VI. 



the polype-cells being formed of the investing soft substance 

 (PI. Y.jig- 9). Yet amidst all these varieties of form, the 

 same essential characters are maintained ; in all there is 

 a skeleton or solid support, and a fleshy or gelatinous 

 substance studded with polypes. 



The Flustra, Eschara, &c. are called Bryozoa, or moss- 

 animals, because their aggregated masses encrust other 

 bodies like moss : their polypes, though, as we have already 

 seen, exceedingly minute, are highly organized, their 

 digestive organs being more complicated than in the other 

 tribes of zoophytes. The polypes of the large calcareous 

 masses that constitute the coral reefs of tropical seas, are of 

 a lower organization, and resemble the common Sea- 

 anemone {Actinia, PI. VI. jig. 11); they are termed 

 Anthozoa, or flower- animals. The lowermost group are 

 the polypes in which digestion is performed by a simple sac 

 or pouch like the common Hydra : these are classed as 

 Hydrozoa, or hydra-like animals.* 



From an analysis of the stony corals, it appears that 

 their composition is very analogous to that of shells. The 

 porcellaneous shells, as the Cowry, are composed of animal 

 gluten and carbonate of lime, and resemble, in their mode 

 of formation, the enamel of the teeth ; whereas the pearly 

 shells, as the Oyster, are formed of carbonate of lime and a 

 gelatinous or cartilaginous substance, the earthy matter 

 being secreted and deposited in the interstices of a cellular 

 tissue, as in bones. In like manner some corals yield 

 gelatine upon the removal of the lime ; while others afford 

 a substance in every respect resembling the membranous 

 structure, obtained by an analysis of the nacreous shells.f A 

 recent elaborate analysis of between thirty and forty species 



* The reader interested in this subject should study that charming 

 and classical work on Comparative Anatomy, Prof. Owen's " Hun- 

 terian Lectures," vol. i. lect. vii. 



f Experiments of Mr. Hatchett. 



