POSTLARVAL DEVELOPMENT. 67 



almost symmetrically disposed with regard to the outer cycle and the ento- 

 ccelic chamber below (plate 2, fig. u). 



The simple character of the entocoelic tentacles on their first appearance 

 is in marked contrast with their bifurcated form in mature polyps. As 

 already mentioned in describing the adult colony, the genus Siderastrea is 

 unique among West Indian corals in that its polyps possess dimorphic 

 tentacles. The entocoelic members consist of a simple stalk, divided above 

 the middle into two smaller halves, each of which is terminated by a white, 

 swollen knob ; the exoccelic tentacles, on the other hand, consist of a simple 

 stem with an apical knob. It was also found that in nearly mature polyps 

 the newly formed entocoelic tentacles of the higher cycles are sometimes 

 simple, while the older members are double (plate 6, fig. 32). 



A few days after the completion of the second (inner) cycle of tentacles 

 in the young polyps, the double character of its members began to assert 

 itself. From certain of the entocoelic interspaces another knob-like pro- 

 tuberance arose beside the first tentacle, which in time became a distinct 

 tentacle with stem and capitulum like the other member (plate 2, fig. 12)- 

 The two moieties thus formed were for some time different in size, but later 

 became equal. The appearance was as if two small tentacles had become 

 intercalated between, but a little central to, the interspaces of the first- 

 formed tentacular cycle. 



The second moiety of the inner tentacles exhibited no constancy in its 

 order of appearance from the different entocceles. In plate 3, fig. 13, the ven- 

 tral axial member has appeared and also the two dorso-laterals, while over the 

 dorsal and ventro-lateral entocceles the simple tentacle alone is present. In 

 four other polyps the dorsal axial tentacle was the first to become doubled, 

 and all the others remained single for a few days longer. In another polyp 

 the additional outgrowth first appeared over the two axial entocceles. 



The new outgrowth was at first wholly independent of the old, and to all 

 appearances the two were to be regarded as distinct tentacles, side by side, 

 each with its own peduncle. Later, however, a single proximal stem was 

 found to arise, which, uniting the two as by a single stalk, raised them some 

 distance above the disc, and the organ thus assumed the form of the ento- 

 tentacles already described in connection with the mature colony (plate 3, 



figs- 15* I 7)- 



The adult bifurcated entoccelic tentacle is thus seen to be primarily 

 represented by two distinct tentacles, which only later are united by the 

 growth of a common peduncle. It represents a unique method of tentacular 

 growth among the Anthozoa. The double character appeared on only a few 



