86 SIDERASTREA RADIANS. 



will be even less in still higher cycles. As the polyps increase in size the 

 forces of growth are less likely to act with the regularity and uniformity 

 which they do in the earlier stages when the polyps are small ; even though 

 in the end the cycles obtain hexameral completion it will be brought about 

 with much individual variation. 



No polyps of S. radians having mesenteries belonging to a fourth cycle 

 have been found, and nothing is yet known as to the normal sequence 

 according to which the cycle is established in other species. 



The chief facts concerning the mesenterial sequence in S. radians may 

 be now summarized : 



1. The six pairs of first-cycle mesenteries (protocnemes) arise as bilat- 

 eral pairs in a regular alternation from one aspect of the polyp to the other. 

 The first four pairs early unite with the stomodgeum, but the two last pairs 

 (v, vi) remain free for a long period. Later, the second and fifth, and the 

 first and sixth mesenteries, on each side, form isocnemic pairs, and the third 

 and fourth pairs constitute the directives. 



2. The six pairs of second-cycle mesenteries arise bilaterally as uni- 

 lateral isocnemic pairs on each side of the polyp, and appear successively 

 in the primary exocceles from the dorsal to the ventral border ; ultimately 

 they become equal and exhibit perfect radial symmetry. 



3. The twelve pairs of third-cycle mesenteries also arise bilaterally as 

 unilateral isocnemic pairs on each side. Normally six pairs appear in a 

 successive manner from the dorsal to the ventral aspect of the polyp, a pair 

 within the dorsal exoccele of each sextant ; then other six pairs appear in 

 the same succession, a pair within the ventral exocoele of each sextant. 

 Generally some of the pairs of third-cycle mesenteries are wanting in mature 

 polyps. 



CORALLUM. 



FIRST CYCLE OF ENTOSEPTA AND SECOND CYCLE OF EXOSEPTA. 



Three or four days after fixation of the larva the skeleton was first 

 observed through the transparent tissues of the living polyp in the form of 

 six small radiating septal upgrowths, practically equal in size. At the 

 same time a narrow peripheral calcareous ring was seen, its outer surface 

 uncovered by the polypal tissues (plate i, fig. 7). The six septa were per- 

 fectly free from one another and from the outer annulus, and arranged at 

 equal distances apart within the six entocoelic chambers, thus alternating 

 with the cycle of six exocoelic tentacles first to arise. Each septum appeared 

 as a somewhat spindle-shaped bar with the upper edge strongly spinous and 

 the lower edge flat and adherent to the glass to which the polyp was affixed. 



