446 Natural History of British Zoophytes. 
Additional Note. 
The only extensive series of experiments we have on the composition of 
Polypidoms are those of Mr Hatchett, published in the Philosophical Transac- 
tions for 1800, and the subsequent progress of zoological and chemical science 
requires a new investigation of the subject. The general results of Mr Hatch - 
ett's experiments are, " that the Madrepores and Millepores are formed of a ge- 
latinous or membranaceous substance, hardened by carbonate of lime, the differ- 
ence consisting only in the mode in which these materials are combined : that 
in the Tubipora, Flustra and Corrallina, some phosphate of lime is mixed with 
the carbonate of lime : that in the Isis the basis is a regularly organized mem- 
branaceous, cartilaginous and horny substance, hardened by carbonate of lime, 
one species only (the Isis ochracea) yielding also a small proportion of phos- 
phate of lime. That the hardening substance of the Gorgonia nobilis is like- 
wise the carbonate of lime, with a small portion of phosphate ; but that the 
matter forming the membranaceous basis consists of two parts, the interior being 
gelatinous, and the external a complete membrane, so formed as to cover the stem 
in the manner of a sheath or tube. That the other Gorgonite consist of a horny 
stem coated by a membrane, which is hardened by carbonate of lime. That the 
sponges are of a nature similar to the horny stems of the Gorgoniae, and only 
differ from these and from each other by the quality of texture. And lastly, 
that the Alcyonia are likewise composed of a soft flexible membranaceous sub- 
stance, very similar to the cortical part of some of the Gorgoniae ; and in like 
manner slightly hardened by carbonate, mixed with a small portion of phosphate 
of lime. 
" From this mass of evidence we collect, in general, that the varieties of bone, 
shell, coral, and the numerous tribe of Zoophytes with which the last are con- 
nected, only differ in composition by the nature and quantity of the hardening 
or ossifying principle, and by the state of the substance with which this princi- 
ple is mixed or connected ; the gluten, or jelly, which cements the particles of 
carbonate or phosphate of lime, and the membrane, cartilage, or horny substance 
which serves as a basis, appearing to be only modifications of the same sub- 
stance, which progressively graduates from a viscid liquid, or gluten, into a ge- 
latinous substance, which again, by increased inspissation, and by the more or 
less perfect degrees of organic arrangement, forms the varieties of membrane, 
cartilage, and horn, which, it seems, form the peculiar differences of the several 
species." Abst. Phil. Trans, i. 25-26. There is a very full analysis of Hatch- 
ett's paper in Thomson's Syst. of Chem. v. p. 559, et seq. The paper itself 
will be found in Phil. Trans, abridg. xviii. p. 706. 
IV. THE BRITISH ZOOPHYTES, DIVIDED INTO FOUR ORDERS. 
FROM what has been said in the preceding chapters, it seems to 
follow that the Polypidom, considered by itself, will prove a fallacious 
guide in any attempt to classify Zoophytes, if it is wished that that 
classification (and other classification we hold next to valueless) 
should give a correct representation of the structure of the animated 
tenants, and distribute these into groups characterized by a unifor- 
mity in the leading traits of their anatomy. It is true, that La- 
