FORMS OF THE POLYPIDOMS. 
53 
condition of the polypidoms of the third section, which contain 
no lime, or very little of it, but are formed of a condensed gela- 
tinous membrane, which resembles horn in every essential pro- 
perty.* 
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 calca- 
reous and horny polypidoms. A curious species of Actinia se- 
cretes a horny basis, the first rudiment of a madrepore but all 
other madrepores are calcareous : the axis of the Astroida is 
sometimes of lime, sometimes of horn, and sometimes of mem- 
brane : the polypidoms of the Hydroida are flexible and horny 
without perhaps any exception ; but there is no hesitation in as- 
serting, that the ascidian tribes fabricate productions, some of 
which are referable to every class that the chemist could devise. 
The reader who is not already familiar with the outward forms 
of our native polypidoms, will most easily obtain a correct idea 
of them, by examining the figures which illustrate this work* 
The very few and insignificant madrepores, or helianthoid poly- 
pidoms, which inhabit the British shores, form either short cy- 
linders or reversed cones, having the apex cupped and starred 
with lamellae, which radiate from the depressed centre to the cir- 
cumference. In the major part of the Astroida, or corticiferous 
polypidoms, there is a central calcareous or horny axis, which 
may be compared to the wood of a tree, and which is formed 
by the successive deposition of layer over layer : this is coat- 
ed or barked round with a living irritable flesh or jelly, thicken- 
ed with calcareous matter, which has usually crystallized in the 
form of spicula. The cells of the polypes are excavated in this 
soft bark, on the surface of which they open by an aperture, 
which is always cut into eight rays disposed in a starred fashion, 
and corresponding to the number of the polype’s tentacula; and this 
aperture can be opened and shut at the pleasure of the inmates. 
In Alcyonium, although an asteroid, there is no solid axis, but 
* See Additional Note. 
-j* It has been doubted whether this horny base is formed by the Actinia, but 
I quite agree with Dr Coldstream, that “ it is secreted by its base, and that it 
is as much part of the animal, in fact its skeleton, as are the calcareous axes of 
Caryophyllaea, Fungia, &c., between which and the true Actinia, it seems to form 
a well-marked link.” — See the Edin. New Phil. Journ. ix. p. 238. 
