486 THE MICROSCOPE. 



MADREPORID^J. Madrepores, Mother-pores, "tree 

 corals," differ from other corals in not having a small 

 skeleton, but one inducted by numbers of small cells for 

 the residence of the living animal : these are very visible 

 in the Madrepore muricata, when the polype is dead and 

 decomposed ; but most distinct in the Oculina ramea, as 

 they are situated at the apparently broken stumps that 

 branch from the trunk of the skeleton (fig. 237, No. 5). 

 Every branch is seen to be covered with multitudes of 

 small pits or dots, scarcely visible to the unassisted vision ; 

 but, when viewed under the microscope, are found to be 

 cells of th$ most beautiful construction, remarkable alike 

 for their mathematical regularity and the exquisite fineness 

 of the materials employed in their composition. A mag- 

 nified drawing of a cell is given at No. 6. The living 

 polypes are exquisite objects for a low power ; their vary- 

 ing colours adding to the richness of the hues covering the 

 bed of the ocean. 



ASTEROIDE.&. A group of Zoophytes received the name 

 of Asteroida from the polypes presenting the form of a 

 star. The fleshy mass is supported by hard calcareous 

 spicula ; some having thick branching processes, perform- 

 ing the part of the skeleton in the human frame. This 

 central internal support is usually denominated the axis. 

 The fleshy mass, or covering, is possessed of sensation, and 

 is ramified by vascular tubes and canals for the sustenance 

 of the animal, and carrying on its vital functions. In- 

 cluded in this genera are Gorgoniadce, Pennatulidoe, Alcy- 

 onidce, Isidce, and Tubiporidce. 



The term coral, or corallum, is restricted to the hard 

 structures deposited in the tissues or by the tissues of the 

 Actinozoa. The whole of this class, however, which are 

 thought to possess a framework called a " coral," are not 

 coralligenous. The Ctenophora, and several species, as the 

 soft-bodied non-adherent Zoantharia, deposit no corallum. 

 There are two kinds of such structure, one called the 

 " sclerobasic " corallum, a true tegumentary excretion, 

 formed by the successive growths from the outer surface of 

 the ocderon ; and another, the " sclerodermic " corallum, 

 deposited within the tissues of the animal. Two prin- 

 cipal modifications of form distinguish the sclerobasis : in 



