Natural History of British Zoophytes. 441 
basis, the first rudiment of a madrepore ;* but all other madrepores 
are calcareous : the axis of the Astroida is sometimes of lime, some- 
times of horn, and sometimes of membrane : the polypidoms of the 
Hydroida are flexible and horny in general, and although ome cal- 
careous species will be found arranged in this order, yet it must be 
acknowledged that, as the animals of these species are unknown, 
or very imperfectly described, the existence of calcareous polypi- 
doms in it is somewhat conjectural ; but there is no hesitation in 
asserting, 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 will illustrate this work. 
The very few and insignificant madrepores, or helianthoid polypi- 
doms, which inhabit the British shores, form either short cylinders 
or reversed cones, having the apex cuped and starred with lamellae, 
which radiate from the depressed centre to the circumference. 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 ; and this is coated or barked round with a living 
irritable flesh or jelly, thickened with calcareous matter, which has 
usually crystallized in the form of spicula. The cells of the poly- 
pes 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 poly- 
pe'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 there is an evident tendency to its formation ; 
the materials lie scattered in the form of spicula in the soft gelati- 
nous centre ; and in the Cydonium these spicula have become 
so numerous, that they impart considerable rigidity to the whole 
mass. 
The polypidoms of the Hydraform and Ascidian zoophytes are con- 
structed on very different plans. Were a botanical nomenclature 
allowable, we might express this difference by saying that those of 
the asteroid polypes are exogenous, seeing they are formed by the 
* 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. 288. 
