52 
FORMS OF THE POLYPIDOMS. 
CHAPTER III. 
The Classifications of Zoophytes. 
The existence of a polypidom is not, as has been already 
mentioned, essential to a polype ; nor does it exercise, when 
present, that great influence over the organization of its archi- 
tects and tenants which might have been anticipated. Thus 
the animal of the madreporous Caryophyllsea does not essential- 
ly differ from the naked Actinia ; and the gelatinous Hydra is 
a true representative of the tenant of the sheathed Sertulariadse 
and Tubularia. No ascidian polype, however, is ever found 
detached, and without a polypidom ; and it is the same with all 
our native Astroida, but, perhaps, the clustered animal-flower 
(Actinia sociata, Ellis , Zoanthus, Cuv.) of the Caribbsean sea, 
might take its place in this tribe with greater propriety than in 
any other. 
In reference to their composition, Polypidoms may be divid- 
ed into i. 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 debateable as that which is 
traced between the sister kingdoms. All are composed of the 
same materials, viz. lime, and a gelatinous or membranaceous 
substance ; and their peculiar characters depend on the differ- 
ent proportions in which the materials are mixed. The calca- 
reous, 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 gelati- 
nous 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 
