7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 31 



M. Gordon comments on some of Duerden's observations on Siderastrcea which 

 corroborate her work (1896) on the ultimate skeletal units of the Madreporarian 

 polyp. She reaffirms that, whatever its mode of origin, whether by secretion 

 or by fission of cellular tissue, the lime-forming layer is at first organic 

 and continuous and that the individual ectoderm cells or nuclear parts exert a 

 determining influence on the origin of the lime-forming skeletal units ("calicoblasts" 

 of the author) in the cuticular product. 



C. Gordon criticises Duerden's conclusions regarding the hexameral arrange- 

 ment of the primary septa in Lophopliyllum proliferum [see Bericht f. 1902 

 Coel. p 20]. He points out that I/., occurring comparatively late (Carboniferous) 

 in the geological history of the Rugosa, is probably modified and therefore 

 not a suitable type in which to establish a primitive arrangement of the septa. 

 In Streptelasma profundum, selected because of its early (Ordovician) geological 

 occurrence, only 4 septa appear in the early stage of the corallum and the 

 secondary septa are added in accordance with the sequence shown for Zaphrentoid 

 corals. The septa which are disposed one on each side of the axial septum 

 on the convex side of the calice in L. are regarded by Duerden as primary, 

 but by the author as accelerated secondary septa. By inversion of some of 

 Duerden's figures of L., a Zaphrentoid type may be produced, at least as 

 regards the adult arrangement of septa and fossula. It is by no means 

 established that in L. the hexameral arrangement is primitive but rather that 

 L. is an accelerated type of a Zaphrentoid coral. - Duerden ( 3 ) replies in 

 detail and concludes that the Rugose corals and the Zoanthid actinians have 

 both a primary hexamerism. The septa in Rugosa and the mesenteries in 

 Zoantheae are never polycyclic as in modern corals and ordinary actinians ; at 

 most there are only two cycles of septa, large entosepta and small entosepta, 

 disposed in such a manner as could only have been produced in polyps with 

 a mesenterial arrangement similar to that of the Zoanthese. The presence of 

 a ventral directive fossula in the Rugosa can be explained by the occurrence 

 within the living rugose polyp of a single ventral siphonoglyph, such as is 

 characteristic of Zoanthid polyps. In addition to L. p., Duerden ( 4 ) has 



definitely determined the presence of 6 primary septa, all equal and at equal 

 distances apart, in Streptelasma rectum, Cyathaxonia cynodon, Hadrophyllum 

 glanSj H. pauciradiatum and Microcyclus discus. Many other species, while 

 not favourable for study at their tips, showed a development of the later septa 

 so closely agreeing with the species above named that in them also the primary 

 septa were hexameral. In no case were there only 4 protosepta. The specimens 

 examined range from the earliest to the latest appearance of the Rugosa. 



Carruthers, working on Zaphrentis 2 sp., Lophopliyllum 1, Oyathaxonia 1, 

 Dibunophyllum 1, Cyclophyllum 1, Gyathopliyllum 1 and Streptelasma 2, finds 

 that the primary septal plan is hexameral in the Carboniferous species as well 

 as in the early Palaeozoic species. 



According to Seurat all the Tuamotu Islands have the same structure and 

 are low-lying, except Makatea which is like an ordinary atoll but elevated 

 to a height of 70 metres. He maintains that Murray's theory is insufficient 

 to explain the formation of atolls, and must be supplemented by Agassiz's 

 conclusion that all the Pacific islands have undergone elevation. This is easily 

 proved in the Gambier group: most of the islands are elevated only a few 

 metres but Makatea is raised 70 m. The most abundant corals are Pavonia, 

 Porites, the Madrepores, Montipores and Fungias. Hydrocorallines, especially 

 Millepora, also play a considerable part in reefbuilding. - - See also CrOSSland, 

 Gardiner( 2 ) and Voe!tzkow( 1 , 2 )- 



