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 some 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 o f 

 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. 



