440 Natural History of British Zoophytes. 
V. The Natural History of British Zoophytes. By GEORGE JOHN- 
STON, M. D. Fellow of the Royal College of Surgeons of Edin- 
burgh. (Continued from p. 24-70 
III.-~ON THE STRUCTURE AND FORMATION OP THE 
POLYPIDOMS. 
THE existence of a polypidom is not, as has been already men- 
tioned, essential to a polype ,* nor does it exercise, when present, 
that great influence over the organization of its architects and ten- 
ants which might have been anticipated. Thus the animal of the 
madreporous Caryophyllsea does not essentially differ from the na- 
ked Actinia ; and the gelatinous Hydra is a true representative of 
the tenant of the sheathed Sertulariadse and Tubularia. No Asci- 
dian polype, however, is ever found detached, and without a poly- 
pidom ; and it is the same with all our native Astroida, but, per- 
haps, the clustered animal-flower (Actinia sociata, Ellis, Zoanthus, 
Cuv.} of the Carribsean sea, might take its place in this tribe with 
greater propriety than in any other. 
* In reference to their composition, Polypidoms may be divided into 
1. the stony or calcareous, 2. the membrane-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 pe- 
culiar characters depend on the different proportions in which the 
materials are mixed. The calcareous, which are hard and inflexible, 
and, when dry, assume a white colour, consist principally of carbo- 
nate of lime, with a small quantity of the phosphate of the same 
earth, and the gelatinous matter which cements them into one co- 
herent mass, is in sparing proportion : that proportion is so greatly 
increased in the polypidoms of the second section, that when the 
earthy ingredients 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 contain no 
lime, or very little of it, but are formed of a condensed gelatinous 
membrane, which resembles horn in every essential property.* 
These diversities in their chemical composition appear to be of 
little value, either in a physiological or systematical point of view, 
for in every order of polypiferous zoophytes, we find calcareous and 
horny polypidoms. A curious species of Actinia secretes a horny 
* See additional Note, p. 446. 
