POSTLARVAL DEVELOPMENT. 75 



according to the hexameral sequence, then the formula must be different from 

 that given above if it has to express the true morphological value of the tenta- 

 cles as entotentacles and as exotentacles. Cyclic incompletion has been 

 shown to be practically always the condition in adult polyps of S. radians; 

 two cycles of eutotentacles are hexamerously complete, but the third cycle of 

 entotentacles consists of a variable number, rarely reaching the required num- 

 ber twelve. Regarded cyclically, some of the exotentacles belong to the third 

 and some to the fourth cycles; but until they form a complete hexameral 

 cycle, it is preferable to consider the exotentacles simply as the outermost 

 tentacles, not as forming a cycle. The morphological formula will thus be 

 6, 6, x, 6 + 6 + x, where x may be any number from one to twelve ; the 

 series 6, 6, x, will represent the number of entotentacles, and 6 + 6 + x 

 the number of exotentacles. The number of exotentacles will vary in the 

 same degree as the number of third-cycle entotentacles. Later, a similar 

 formula is established for the septa. 



The different tentacular cycles in the adult polyps of Siderastrea are 

 much more widely apart than in the larval polyps (cf. plate 6, fig. 31 ; 

 plate 3, fig. 17). In the latter the closeness of the cycles recalls the condition 

 characteristic of the majority of anemones and corals. 



The principal facts concerning the development of the tentacles of S. 

 radians may be now summarized. 



1. The first tentacles to appear consist of a cycle of six members, one 

 from each of the six primary exocceles. They arise simultaneously, are 

 equal in size, and remain simple throughout. 



2. Shortly afterwards six smaller tentacles arise, forming a second cycle, 

 situated internally to the first, and communicating with the six primary 

 entocceles. They may develop either simultaneously or in successive pairs 

 from the dorsal to the ventral aspect. They are not symmetrically situated 

 as concerns the mesenterial chambers. Later, another entoccelic tentacle 

 appears alongside each of the first members, and then a common peduncle is 

 formed which elevates the two moieties so as to form the single bifurcated 

 tentacle of the adult. 



3. The second cycle of mesenteries having appeared, a new exotentacle 

 arises from each additional exocoelic chamber within the six primary systems 

 of the polyp. In general the succession is from the dorsal to the ventral 

 systems, and the new tentacle is on the ventral aspect of the older exoten- 

 tacle belonging to the same system, but many variations occur. The six 

 new exotentacles arrange themselves in the outermost or second cycle, which 

 thus consists of twelve members, equal in size, and all simple. 



