196 



ZOOLOGY 



SECT. 



arise, diverging slightly from one another as they grow, and 

 separated by tolerably wide intervals. The distal ends of the 

 polypes then grow out into horizontal expansions or platforms 

 (pi), formed at first of ectoderm and mesogloea only, but 



finally receiving prolongations of 

 the endoderm. The platforms ex- 

 tend, come in contact with one 

 another, and fuse. In this way 

 platforms of considerable extent are 

 formed (A, pi), uniting the polypes 

 with one another. From the upper 

 surfaces of the platforms, between 

 the older polypes, new buds arise, 

 and in this way the colony tends to 

 assume the form of an inverted 

 pyramid, the number of zooids, and 

 consequently the diameter of the 

 colony, increasing pari passu with 

 the vertical growth of the latter. 

 The skeleton of this remarkable 

 coral will be referred to hereafter. 

 Although the general structure of the individual polypes 

 of the Actinozoa is, as mentioned above, very uniform, the varia- 

 tions in detail are numerous and interesting, especially among 



FIG. 146. -Corallium rubrum, por- 

 tion of a branch. (From Claus, 

 after Lacazc-Duthiers.) 



*-. 



FIG. 147. Astraea pallida, the living colony. (After Dana.) 



the Actiniaria. One of the most important points to consider 

 is the arrangement of the mesenteries. In Edwardsia (Fig. 150), 

 a genus which burrows in sand instead of attaching itself to 

 rocks, &c., there are only eight mesenteries (B) the usual two 

 couples of directives, and two others on each side of the vertical 

 plane, having their longitudinal muscles directed ventrally, and 

 therefore not arranged in couples. The adult Edwardsia thus 



