POLYPI. 167 



are crossed by radiating plates, adapted to the 

 form and number of the tentacula. In Mille- 

 pores 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 net- 

 work. 



The material of which this axis, to which the 

 polypes are attached, is composed, is of various 

 kinds. Sometimes it is horny, flexible, and 

 elastic, corresponding in its nature to animal 

 membrane : at other times it is hard and calca- 

 reous, being composed principally of carbonate 

 of lime, with a small quantity of the phosphate ; 

 the proportion of this latter ingredient varying 

 in different species. In all cases the particles 

 of calcareous matter are united together by some 

 portion of animal substance which may be ob- 

 tained by dissolving out the former by an acid. 

 We always find the materials arranged in con- 

 centric layers, indicating 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 dis- 

 posed alternately, composing a jointed structure, 

 which some have fancied might be considered 

 as making an approach to an articulated skele- 

 ton ; for it is capable of considerable flexion, 

 and readily yields to the impulse of the waves, 



