ACTINOZOA. 



173 



series of concentric circlets, each of which, save 

 the second, includes twice the number of tentacles 

 proper to its predecessor. Thus, the first circlet 

 contains 6 tentacles, the second 6, the third 1 2, 

 the fourth 24, the fifth 48, and the sixth 96. In 

 Antipathes and some other polypes never more 

 than the first six tentacles arise. Among certain 

 other Zoantharia, one or more tentacles are occa- 

 sionally aborted, and hence the somewhat puzzling 

 numerical proportion of these organs noticed in 

 slightly abnormal forms of this group. 



Should the young polype give rise to a calcareous 

 corallum, the early stages of its deposition consist 

 chiefly in the formation of spicules which, at first 

 small and detached, gradually increase in size and 

 coalesce in a greater or less degree to form the 

 various structures whose nature has elsewhere been 



explained. 



Where a septal apparatus occurs, the develop- 

 ment of its several elements follows the same 

 definite law by which the number of the mesen- 

 teries and tentacles is determined. 



The young Zoantharian has at first six septa, 

 the Rugose Coral four, equidistant from one 

 another, and separating similar loculi. In a few 

 genera only can a smaller number of initial septa 

 be detected. But among the great majority of 

 Zoantharia, the typical grouping of the septa is 

 hexameral ; among the Rugosa, tetrameral. 



In some Zoantharia the number of the septa 

 never exceeds six, but, more frequently, new septa 

 appear midway between those first formed, at the 

 lower portion of the theca. These gradually grow 

 inwards, at the same time increasing in height. 

 Thus the primarv loculi become divided into 



