68 SIDERASTREA RADIANS. 



of the most forward larval polyps, and, for the time they were under obser- 

 vation, was in no instance established for all the six members. 



For another period of six weeks or two months no further tentacular 

 increase took place, though in the meantime the second cycle of mesenteries 

 had made its appearance. 



So far, four distinct stages are recognizable in the development of the 

 twelve prototentacles of Siderastrea, each stage separated by marked intervals 

 of rest. They are represented diagrammatically in the figures a-c on p. 72 

 as they would appear when fully completed. 



1. The stage with six exocoelic tentacles, forming a single cycle. The 

 members are equal and appear simultaneously. 



2. The stage with twelve simple tentacles, constituted of an outer cycle 

 of six large exotentacles and an inner alternating cycle of six small ento- 

 tentacles. The latter appear either simultaneously or successively in an 

 irregular sequence. They are not situated midway over the mesenterial 

 chambers. The four lateral members are disposed a little towards the dorsal 

 or ventral half of the interspaces, while the directive members are somewhat 

 to one side or the other of the principal oral axis of the polyp. 



3. Six additional members of the inner entocoelic cycle appear, so that 

 the cycle is constituted of twelve independent tentacles in six pairs. They 

 arise in no definite sequence. 



4. A single outgrowth arises over the middle of each entoccele and forms 

 a common peduncle for each pair of entotentacles. Thus the tentacular 

 outgrowths from each entoccele come to resemble a bilobed adult tentacle. 



The appearance of the exocoelic tentacles in advance of the entocoelic in 

 Siderastrea radians is unique in the tentacular development of Zoantharian 

 polyps according to the researches of I/acaze-Duthiers, Faurot, von Koch, 

 Appellof and others, who have made us acquainted with the post-larval 

 development of many species of anemones and corals. In the corals Caryo- 

 phyllia cyathus, Manicina areolata, and Favia fragum the twelve proto- 

 tentacles, constituting an inner and outer cycle, appear simultaneously, or 

 at most there is only a short interval between one and the other ; but 

 wherever such an interval occurs, the inner entocoelic cycle appears in 

 advance of the outer exocoelic, and the entocoelic members are larger than 

 the exocoelic (entacmseous). In general it will be found that the production 

 of other entocoelic structures tentacles and septa in corals is in advance 

 of the exocoelic. 



In actinian larvae it is more usual for eight tentacles to appear first, 

 arranged either as a single cycle or as an inner and outer cycle of four each. 



