32 Coelenterata. 



are formed. Each fuses with the central part of the corresponding second-cycle 

 exoseptum, these thereby losing their individuality. 12 members of a temporary 

 third cycle are situated within the exocoels between the primary and secondary 

 pairs of mesenteries and represent the bifurcated extensions of the 6 primary 

 exosepta. The original second cycle exosepta thus become the third exocoelic 

 cycle, their place having been taken by the new (permanent) second cycle of 

 entosepta. A new third cycle of 12 (or less) septa arises, on the appearance 

 of the third cycle mesenteries, in a manner similar to that followed by the 

 second permanent cycle. New entosepta appear in the entocoels of the third 

 cycle mesenteries, and the bifurcations of the third cycle exosepta then become 

 the exosepta of the fourth cycle. Exosepta are present at each cyclic stage in 

 the growth of the corallum, alternating in position and corresponding in number 

 with the sum of the entosepta; they never become entosepta but always con- 

 stitute the outermost cycle of shorter septa. Until the adult condition is reached 

 the exosepta are the temporary predecessors of the entosepta, their develop- 

 mental relationships are closely comparable to those of the ento- and exo- 

 tentacles. The law of substitution, first discovered by Lacaze-Duthiers for the 

 tentacles of Hexactinise, is thus found to hold also for the septa. The cycles 

 of septa and mesenteries represent so many distinct phases of growth at inter- 

 vals all round the polyp. With the exception of the first, the members of each 

 cycle follow a dorso- ventral succession, display a bilateral symmetry for some 

 time and ultimately assume an approximately radial plan. 



Duerden( 4 ) describes the anatomy, histology and postlarval development of 

 SiderastrcBa radians. S. differs from most coral- and actinian-polyps in that 

 its tentacles are arranged widely apart and thus occupy a broad zone 

 of the disc leaving only a small smooth central area with the mouth in the 

 middle. They are arranged only approximately in cycles. The members of 

 the inner cycles are bifid and entoccelic and, as seen externally, each appears 

 to originate over the central termination of the septum to which it belongs. 

 The members of the outer cycle are simple and exoccelic. Large nematocysts 

 are restricted to the solid apical knobs of the tentacles. The tentacles some- 

 times appear as unstalked spheroidal tubercles sessile on the disc, the stem of 

 the tentacle having become part of the discal wall. S. is apparently the only 

 Madreporarian genus with dimorphic tentacles. The polyps are various shades 

 of brown, the colour being entirely due to the presence of numerous zooxan- 

 thellas or unicellular algae in the endoderm. New polyps arise as intercalary 

 buds over the general surface of the colony and generally there is also rapid 

 budding around the margin. Occasionally much enlarged calices are seen under- 

 going fission. (Fissiparous gemmation, see Bericht for 1903 Coel. p 18.) 

 In apparently dead parts of a colony new polyps may arise, possibly as buds 

 from living remnants of the original polyps. The individual polyps are free 

 from one another except at the surface of the colony where they are all in 

 communication by means of intermesenterial apertures. The column wall is 

 very thin (.07 mm) and is composed of supporting cells and clear gland-cells 

 while small nematocysts sparingly occur. There is no trace of ectodermal 

 musculature in the column, but weak radial fibrils are present in the disc. There 

 is no evidence of external ciliation. There is a delicate circular entodermal 

 muscle layer, but no concentration into a sphincter and in consequence the 

 polyps are unable to fold the column over the disc during contraction. There 

 are intercellular spaces filled with some mucoid substance in the entoderm 

 of most parts of the polyp. The stomodseum is uniformly ciliated, there are 

 no permanent grooves and no histological modification suggesting a siphono- 



