74 SIDERASTREA RADIANS. 



stances it was not possible to say whether the new exotentacles appeared in 

 advance of the entotentacles, but from the order actually observed in the 

 first three cycles it may be assumed that such was the sequence. It will be 

 observed that the new tentacular pairs are situated on the dorsal aspect of 

 the four sextants, along with one pair on the ventral side of the right middle 

 sextant. 



New exotentacles and entotentacles have been found to arise shortly after 

 the appearance of new mesenterial pairs. Therefore, if the third-cj'cle mesen- 

 teries develop normally with the regularity outlined on p. 83, a single ento- 

 tentacle and exotentacle would be expected to appear on the dorsal aspect of 

 each sextant, and then the corresponding ventral series would commence, 

 both starting with the dorso-lateral systems and proceeding to the ventro- 

 lateral. In a colony where the polyps are so closely arranged as in Sideras- 

 trea^ however, the later growth rarely proceeds according to such a regular 

 law, so that very little importance can be attached to the order of appearance 

 of the tentacles making up the outer cycles. All the studies on corals seem 

 to indicate that the mesenteries, tentacles, and septa develop conformably to 

 a regular plan for the first and second cycles, but that the regularity is 

 frequently departed from in the later stages. 



If the cyclic hexameral plan of the tentacles in S. radians were com- 

 pleted the formula would be 6, 6, 12, 24. The first three cycles would be 

 entotentacles and the last exotentacles ; the members of any one cycle would 

 be equal in size, but those of the inner cycles somewhat larger than those of 

 the outer cycle. The entotentacles would have appeared in a fairly regular 

 sequence, the inner cycle first, the middle cycle next, and the third cycle 

 last ; but it is altogether otherwise with the fourth cycle, constituted only of 

 exotentacles. Six of its members appeared a little in advance of the first 

 cycle of entotentacles; other six a little in advance of the second cycle of 

 entotentacles ; and twelve more in advance of the third cycle of entotentacles. 

 The exotentacles at each stage form a complete cycle ; first a cycle of six, 

 then a second cycle of twelve, which afterwards becomes a third cycle, and, 

 lastly, a third cycle of twenty -four, which afterwards becomes a fourth cycle. 

 In this way they always constitute the last or outermost cycle for the time 

 being. At the termination of each stage the number of outer simple exoten- 

 tacles equals the sum of the bifurcated entotentacles making up the inner 

 cycles ; but, until the adult stage is reached, as a cycle they occupy the 

 position which later will be occupied by the entotentacles. 



When, however, the hexameral cyclic plan is not complete in the adult, 

 that is, where the last cycle of entotentacles commenced is not completed 



