436 FORMS OF THE POLYPIDOMS. 



contain no lime, or very little of it, but are formed of a con- 

 densed 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 spe- 

 cies of Actinia secretes 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 membrane : the polypidoms of the Hy- 

 droida are flexible and horny without perhaps any exception ; 

 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 illus- 

 trate this work. The very few and insignificant madrepores, 

 or helianthoid polypidoms, which inhabit the British shores, 

 form either short cylinders or reversed cones, having the apex 

 cupped 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 cen- 

 tral calcareous or horny axis, which may be compared to the 

 wood of a tree, and which is formed by the successive depo- 

 sition of layer over layer : 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 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 



* 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. 



