ADULT COLONY. II 



The hexameral cyclic arrangement of the tentacles is by no means easy 

 to establish, and would be almost impossible to ascertain without anatomical 

 aid or assistance from the septa below. According to the usually accepted 

 cyclic formula for hexactinian polyps employed in systematic works, S. 

 radians would be said to have three cycles complete and a fourth incomplete, 

 and the cyclic formula would be 6, 6, 12, x, where x may be any number 

 from i to 24. 



On the living polyp six bifurcated tentacles can usually be distinguished 

 as constituting an inner or primary cycle, which is widely separated from 

 the other cycles ; the remaining tentacles are more closely disposed and their 

 arrangement is somewhat obscure, simple and double members being inter- 

 mingled. Among them a second cycle of six bilobed tentacles can sometimes 

 be recognized, alternating with the first and remote from it. Beyond this 

 there is no true hexameral plan determinable. In most polyps a few mem- 

 bers of a third cycle of bilobed tentacles are present, but the full complement of 

 twelve entotentacles, necessary by the laws of actinian symmetry to complete 

 the alternation with the two previous cycles of six each, appears never to be 

 developed. The outer simple exoccelic tentacles correspond in number with 

 the sum of the members of the primary, secondary, and tertiary entotentacles 

 with which they alternate, but some belong to the third cycle and some to 

 the fourth. Only the first and second cycles are completely hexameral ; 

 the third and fourth are incompletely so. 



In transverse sections through the polyps (plate 6, fig. 34) it is found 

 that the first and second cycles of mesenteries consist of six pairs each, but the 

 third cycle rarely or never contains the next number in the hexameral sequence, 

 namely, twelve. The number of entoccelic chambers corresponds with the 

 number of exoccelic chambers, therefore the number of entotentacles will 

 correspond with the number of exotentacles, as a tentacle communicates with 

 each mesenterial chamber. Whatever number of entotentacles be lacking 

 to complete the third cycle of 12, a like number of exotentacles will be 

 wanting to complete the fourth cycle of 24. Thus, considered morpho- 

 logically, the tentacular formula will be 6, 6, x, 6 -f- 6 -f x, where x will 

 be the same in the two cases, and may be any number from i to 12 ; the 

 series 6, 6, x will represent the number of entotentacles, and 6 -f- 6 -f x the 

 number of exotentacles. A polyp with 38 tentacles will have the morpho- 

 logical formula 6, 6, 7, 19 ; a polyp with 44 tentacles the formula 6, 6, 10, 22. 



Where, in mature polyps, the hexameral sequence is incomplete, it seems 

 preferable to employ the morphological formula as compared with the ordinary 

 cyclic formula. In the first we have the true nature of the tentacles indicated ; 



