POLYPI. 151 



Madrepores these ilepressious are crossed by radi- 

 ating plates, adapted to the form and number of 

 the tentacida. In Millepores the cells are closer 

 and more minute, and exhibit none of these star- 

 like radiations. In some species the plates have 

 more of a parallel arrangement ; and in others they 

 form a network. 



The material of which this axis, to which the 

 polypes are attached, is composed, is of various 

 kinds. Sometimes it is horny, flexible, and elas- 

 tic, corresponding in its nature to animal mem- 

 brane : at other times it is hard and calcareous, 

 being composed principally of carbonate of lime, 

 with a small quantity of the phosphate ; the pro- 

 portion of this latter ingredient varying in diflerent 

 species. In all cases the particles of calcareous 

 matter are united together by some portion of 

 animal substance, which may be obtained by dis- 

 solving out the former by an acid. We always find 

 the materials arranged in concentric layers, indi- 

 cating that their deposition has been successive; 

 and the surface is marked by longitudinal lines, 

 corresponding to the figure of the animal covering 

 of flesh. Sometimes the stem consists of horny 

 and calcareous parts disposed alternately, compos- 

 ing a jointed structure, which some have fancied 

 might be considered as making an approach to an 

 articulated skeleton ; for it is capable of consider- 

 able flexion, and readily yields to the impulse of 

 the waves, without the risk of being broken. This 

 is the case with the Isis hippuris, commonly known 

 by the name oi jointed coral. (Fig. 68.) There is, 

 in short, hardly any possible combination of these 

 parts which does not occasionally occur amidst the 



