72 



SIDERASTREA RADIANS. 



like the members of the first-cycle entotentacles. In general, the sequence 

 of the entotentacles would follow that of the mesenteries with which they are 

 associated, although in plate 3, fig. 17, the growth within the middle sextants 

 is in advance of that of the dorsal sextants. 



The completed system of tentacles would consist of: (a) Six bifurcated 

 entotentacles constituting the first cycle ; {6) six alternating bifurcated ento- 

 tentacles forming the second cycle ; (c) twelve alternating exotentacles, which 

 have appeared at two wholly distinct periods, making up the third cycle. 



The developmental relations between the six second-cycle entotentacles 

 and the twelve third-cycle exotentacles are somewhat different in Siderastrea 



X I X 



Fig. 7. a-^, series of diagrams illustrating the order of appearance and relationships 

 of the lirst three cycles of tentacles. The numerals I and II indicate the orders 

 of entotentacles and X the exotentacles. 



from those hitherto met with in actinians. Lacaze- 

 Duthiers (1872) first found that after the completion 

 of the first two cycles of tentacles (six entotentacles 

 and six exotentacles), the tentacles of the third cycle 

 do not arise peripherally, alternating with the mem- 

 bers of the first and second cycles, as would be 

 naturally supposed, but that six new pairs are insinuated between the two 

 primary cycles at six intervals only. One member of each of the six new 

 pairs increases in size more than the other member, and ultimately consti- 

 tutes the second cycle of tentacles (entotentacles), displacing the primary 

 second cycle of exotentacles ; the other member of each new tentacular pair 

 attains the same size as the displaced exotentacles, and along with them 

 forms the third cycle of twelve tentacles. The third cycle is thus formed of 

 six tentacles from the original second cycle and six tentacles which are later 

 formations ; the members of the permanent second cycle are all new forma- 

 tions which have displaced the older second-cycle members. Lacaze-Duthiers 

 expressed these relationships under the term " law of substitution." 



