ACTINOIDS. 7 



ways corresponding to the cavity of the chambers, may be there- 

 fore said to ride this second set of partitions arising just in the 

 centre of the chambers. 



The third and highest order of Polyps is that of the Halcyo- 

 noids. Here the partitions are reduced to eight ; the tentacles, 

 according to the invariable rule, agree in niunber with the cham- 

 bers, but have a far more highly complicated structure than in 

 the lower Polyps. Some of these Halcyonoids deposit limestone 

 particles in their frame. But the tendency to solidify is not lim- 

 ited to definite points, as in the Madreporians. It may take place 

 anywhere, the rigidity of the whole structure increasiag of course 

 in proportion to the accumulation of limestone. There are many 

 kinds, in which the axis always remains soft or cartilaginous, 

 while others, as the so-called sea-fans for instance, well known 

 among the corals for their beauty of form and color, are stiff 

 and hard throughout. Whatever their character in this respect, 

 however, they are always compound, living in communities, and 

 never found as separate individuals after their early stages of 

 growth. Some of those with soft axis lead a wandering Ufe, 

 enjoying as much freedom of movement as if they had an indi- 

 vidual existence, shooting through the water like the PennatulaB, 

 well known on the California coast, or working their way through 

 the sand like the Renilla, common on the sandy shores of our 

 Southern States. 



ACTINOIDS. 



Actinia, or Sea-Anemone. (Metridium marginatum Edw.) 



Nothing can be more unprepossessing than a sea-anemone when 

 contracted. A mere lump of brown or whitish jeUy, it lies like 

 a lifeless thing on the rock to which it clings, and it is difficult to 

 believe that it has an elaborate and exceedingly delicate inter- 

 nal organization, or will ever expand into such grace and beauty 

 as really to deserve the name of the flower after which it has been 

 called. Pigs. 2, 3, 4, and 5, show this animal in its various stages 



