POSTLARVAL DEVELOPMENT. 69 



These correspond with the eight interspaces between the four pairs of 

 Edwardsian mesenteries ; later, as the fifth and sixth pairs of protocnemes 

 increase in size, other two pairs of tentacles arise, and the whole become 

 arranged in two cycles of six each. Of the primary eight tentacles in 

 actinians six are the entocoelic members, and they early assume predomi- 

 nance over the exocoelic series (e. g., Lebrunia, 1899). 



Clearly the number of external tentacles first appearing is determined 

 by the number of internal mesenterial chambers already formed, seeing that 

 the cavities of the former are continuations of the latter. Where, as in the 

 actinian Lebrunia (1899), only four pairs of mesenteries are present when 

 the tentacles make their appearance, it is manifest that only eight tentacles 

 will be developed, and that the other four will be formed only when the 

 growth of the fifth and sixth mesenterial pairs has given rise to four 

 additional mesenterial chambers. In the species of coral larvae so far fully 

 investigated the six pairs of primary mesenteries four pairs of macrocnemes 

 and two pairs of microcnemes are fully established at or shortly after the 

 time of fixation and before the tentacles begin to protrude; hence six or 

 twelve tentacles arise simultaneously, one from each mesenterial chamber. 



The long interval which elapses before the more complex entocoelic 

 tentacles are fully formed may perhaps account for the appearance in Side- 

 rastrea of the exotentacles in advance of the entotentacles. The young 

 polyp after fixation has to provide for itself, and for this purpose its weapons 

 of offense and defense, represented by the tentacular nematoblasts, are 

 necessary from the beginning. 



The origin of the bifurcated entotentacles from two independent moieties 

 which appear at different times and are later elevated upon a common 

 peduncle is a second unique feature in tentacular development. Obviously 

 the presence of a double battery of nematocysts on each tentacle, in place of 

 a single battery, will be of advantage to the polyp ; and the growth of one 

 moiety in advance of the other is probably to be taken as indicating an 

 ancestry in which only one was present. 



SECONDARY EXOTENTACLES. 



Much interest attaches to the manner of appearance of the tentacles 

 beyond the prototentacles, as, from the exceptional sequence of the first two 

 cycles, it is impossible to predict from the known development of other forms 

 how or where the next members will arise. One of the earliest stages is 

 represented by the fully expanded disc in plate 3, fig. 15. The second- 

 cycle mesenteries are seen stretching along the disc, decreasing in size 



