POSTLARVAL DEVELOPMENT. 71 



the older and the newer tentacles are seen to communicate only with the 

 exocceles, and the second-cycle entocceles are for a time without any tentacular 

 outgrowths. The cycle of exotentacles when complete would at this stage 

 consist of twelve members, of which six are primary and six are later forma- 

 tions, the latter appearing after the second cycle of mesenteries (fig. 

 7, d, p. 72). 



The tentacular development of S. radians thus conforms with the law 

 of substitution established by Lacaze-Duthiers (1872, '73) and Faurot (1895) 

 for actinians exotentacles remain exotentacles, and throughout the growth 

 of the polyp they continually change their relationship to the entotentacles 

 in such a manner as always to constitute the outermost cycle, and all are 

 uniform in size. 



As in the case of the two cycles of primary tentacles the secondary 

 exotentacles appear in advance of the corresponding secondary entotentacles, 

 so that now the cycle of exotentacles contains double the number of members 

 of the cycle of entotentacles. In the several species whose development was 

 carried thus far by Lacaze-Duthiers and by Faurot the exoccelic and entoccelic 

 members appeared together along with a new pair of mesenteries, or the ento- 

 tentacles were in advance of the exotentacles ; later the entotentacles became 

 larger than the exotentacles. 



SECOND CYCLE OF ENTOTENTACLES AND THIRD CYCLE OF EXOTENTACLES. 



Clearly the stage beyond that represented in plate 3, fig. 16, will be one 

 in which the tentacles begin to make their appearance over the second-cycle 

 entocceles. Such a condition is represented in plate 3, fig. 17, taken from the 

 same polyp as plate 3, figs. 15 and 16, when about a fortnight older than the 

 stage of plate 3, fig. 16. It is also the oldest stage reared. From the middle 

 second-cycle entoccele on each side a simple tentacle has arisen, intercalated 

 a little beyond the primary cycle of entotentacles, but yet within the cycle 

 of exotentacles. The two tentacles are the first representatives of the second- 

 cycle tentacles of the adult polyps, and the greatest interest attaches to the 

 fact that they have displaced, as it were, some of the members of the second 

 cycle (exotentacles), so that they now form a third cycle. 



If the development of the polyps had been continued, there is every 

 reason to suppose that a new tentacle would have appeared over each of the 

 dorsal second-cycle entocceles. Also in the further growth of the two ventral 

 systems another exotentacle would have appeared on each side, and then a 

 tentacle over the second-cycle entoccele, just as in the middle systems; then 

 all the simple second-cycle entotentacles would in time have become bifurcated 



