64 SIDERASTREA RADIANS. 



As regards the tentacles, six primary exotentacles appeared simulta- 

 neously, shortly after fixation (plate i, fig. 6), followed by an interruption 

 of two or three weeks before the entotentacles began to arise. These were 

 at first simple outgrowths of the disc, and a long interval took place before 

 a second moiety appeared in connection with each and thus established the 

 mature form (plate 2, fig. n). The longest interruption, however, was after 

 the completion of the primary twelve tentacles, for in the most forward polyps 

 about three months elapsed before additional members appeared, when those 

 of another cycle began to arise in a successive manner. 



Of the septa the six entoccelic representatives appeared as a complete 

 cycle at a very early stage (plate i, fig. 7), followed shortly by the six alternat- 

 ing exocoelic members ; these latter were developed in some instances as a 

 complete cycle, but usually in successive pairs (plate 2, figs. 8, 9). Without 

 any marked rest other fragments or nodules were added to each system, but 

 in such a way as to render it doubtful as to which cycle they belonged (plate 

 2, fig. 12). On the whole, the growth of the septa was more continuous 

 than that of either the mesenteries or the tentacles. 



Undoubtedly the most conspicuous interruption for all the systems of 

 organs was that between the protocnemic and metacnemic stages, and the 

 great differences between the manner of appearance of the two series of 

 mesenteries, septa, and tentacles further establishes this as the most impor- 

 tant ontogenetic and phylogenetic interval in madreporarian development. 

 Its significance is discussed later. 



The time of appearance of the organs varied somewhat in different polyps, 

 sometimes even to the extent of several weeks ; but with regard to the actual 

 order followed no important differences were observed in the mesenteries and 

 septa, though less constancy prevailed amongst the tentacles. Where devel- 

 opment was not in complete cycles the succession was bilateral, proceeding 

 from the dorsal to the ventral borders of the polyp. Contrary to what is 

 generally assumed, bilaterality, not radiality, was a conspicuous feature of 

 the growth from beginning to end ; and in most of the systems of organs 

 the bilateral symmetry prevailed for lengthened periods, even where the 

 radial condition was ultimately assumed. 



Many of the young polyps were preserved at different stages while still 

 adhering to their original pieces of glass and were prepared for microscopic 

 examination as a whole. The process was as follows : After narcotization 

 by means of magnesium sulphate or menthol the polyps were killed by 

 pouring a solution of formaldehyde over them, and, still adherent, they were 

 transferred to alcohol, afterwards stained, dehydrated, cleared, and mounted 



