126 THE MECHANICAL FUNCTIONS. * 



sions in the surface of the calcareous axis, which affords them 

 some degree of protection. In Madrepores these depres- 

 sions are crossed by radiating plates, adapted to the form 

 and number of the tentacula. In Millej)ores the cells are 

 closer and more minute, and exhibit none of these star-like 

 radiations. In some species the plates have more of a pa- 

 rallel 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 calcareous, 

 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 obtained by dis- 

 solving out the former by an acid. We always find the ma- 

 terials arranged in concentric layers, indicating that their 

 deposition has been successive; and the surface is marked 

 by longitudinal lines, corresponding to the figure of the ani- 

 mal covering of flesh. Sometimes the stem consists of horny 

 and calcareous parts disposed alternately, composing a jointed 

 structure, which some have fancied might be considered as 

 making an approach to an articulated skeleton; for it is ca- 

 pable of considerable flexion, and readily yields to the im- 

 pulse of the waves, without the risk of being broken. This 

 is the case with the his hipj)uris, commonly known by the 

 name oi jointed coral. (Fig. 68.) There is, in short, hardly 

 any possible combination of these parts which does not oc- 

 casionally occur amidst the infinite diversities of condition 

 displayed in this department of the animal creation. 



These structures are generally attached to submarine rocks 

 by an expansion of the base into a kind of foot, or root, 

 which has a strong power of adhesion. In this respect, 

 therefore, as in so many others, these animals preserve an 

 analogy with plants. 



It has been ascertained that, in a great number of instances, 

 these fixed zoophytes are multiplied, like the sponge, by the 



