POSTLARVAL DEVELOPMENT. 73 



In Siderastrea the second cycle for a time consists of twelve exotentacles, 

 six of which are primary formations and six are later outgrowths. The latter 

 appear in advance of the corresponding entotentacles, not along with them, as 

 seems to be the case in most actinians. Afterwards, however, a new cycle of 

 six entotentacles is intercalated between the original first and second cycles, 

 displacing the latter to the third cycle, and itself constituting the permanent 

 second cycle. The law of substitution thus holds for Siderastrea as for 

 actinians, though the relative time of appearance of the entotentacles and 

 exotentacles differs from that usually followed. Faurot (1895) shows that in 

 Ilyanthus partlienopeus the entotentacle from a new mesenterial pair arises 

 in advance of the exotentacle, exactly the reverse of that in Siderastrea. 



THIRD CYCLE OF ENTOTENTACLES AND FOURTH CYCLE OF EXOTENTACLES. 



As the larval polyps were not reared beyond the beginning of the 

 three-cycle stage, it will be necessary, in order to complete the survey of the 

 tentacular development, to have recourse to bud polyps of a colony. From 

 what has been established above it is manifest that the problem is not to 

 determine the manner according to which the fourth cycle of the adult polyp 

 will arise, but how a third cycle of entotentacles originates and is inter- 

 calated between the previously formed second and third cycles. The exo- 

 tentacles as a cycle are shown to have no ordinal significance in studies of 

 tentacular sequence ; this importance belongs to the entotentacles. Regarded 

 cyclically the exotentacles are but temporary predecessors of the entotentacles 

 of the adult polyp. 



On any colony are numerous polyps in which the tentacles vary from 

 twenty-four to nearly forty-eight. In the polyp represented on plate 6, fig. 

 32, there are three third-cycle entotentacles, each with a corresponding 

 exotentacle. The former are yet at an early stage of development, only one 

 moiety of the tentacle having appeared. In fig. i, p. 12, is represented 

 diagrammatically the tentacular plan of the polyp of which the mesenterial 

 system is given in plate 6, fig. 34. Along with each of the five pairs of 

 third-cycle mesenteries (ill) have appeared two corresponding tentacles, and, 

 comparing with fig. 3, p. 26, one of the tentacles is seen to be entocoelic (in) 

 and the other exocoelic (x). The new entotentacles, in, are situated beyond 

 the second cycle of entotentacles, n, but within the outermost exocoelic 

 cycle ; that is, they represent the commencement of a third cycle of ento- 

 tentacles, and the exotentacles are now being relegated to a fourth cycle. 

 At this stage, therefore, some of the exotentacles may be considered as 

 belonging to the third cycle and some to the fourth cycle. Under the circum- 



