﻿duerden] morphology of coral polyps J ! * 



SUMMARY 



The studies thus briefly outlined enable the relationships of madre- 

 porarian coral polyps to be determined with much precision. With 

 the exception of the characteristics dependenl on the presence of a 

 skeleton, they present no feature which separates them from ordinary 

 hexameral actinians. The development and arrangement of the 

 mesenteries and the tentacles, in both the protocnemic and metac- 

 nemic stages, are the same in both groups. In the ahsence of siphono- 

 glyphs from the stomodaeum, and of lateral ciliated bands from the 

 mesenterial filaments, coral polyps differ from the great majority 

 of anemones, but some of the lower actinians are without siphono- 

 glyphs and have but simple filaments. Modern Actiniaria (exclud- 

 ing the Ceriantheas and Zoantheje) and Madreporaria constitute a 

 single group, one section of which forms a skeleton while this is 

 absent in the other. On the other hand the Paleozoic rugose corals 

 diverge from modern corals after the formation of the six primary 

 septa, their septa are then added in the same sequence as are the 

 mesenteries in the Zoantheae ; further, the single ventral siphonoglyph 

 of the zoanthids was probably present in the rugose polyp, being now 

 represented in the skeleton by the fossula. The Rugosa and Zoan- 

 theaa undoubtedly constitute a common group of skeleton-forming 

 and skeletonless polyps, just as do the modern Madreporaria and 

 ordinary hexamerous Actiniaria. 



Modern Madreporaria may be defined as follows : 



Madreporaria 



Anthozoa of which the polyps are either simple or colonial. The 

 basal ectoderm gives rise to a continuous external calcareous skele- 

 ton, usually consisting of basal, peripheral, and radial elements. 

 Colonial polyps are in communication at the proximal termination 

 of the column, and sometimes by basal canals ramifying through 

 the skeleton. Tentacles in alternating cycles, often with a knobbed 

 or swollen apex. Stomodaeum smooth or ridged, without gonidial 

 grooves or siphonoglyphs. The mesenteries include a primary cycle 

 of six pairs, appearing successively in bilateral pairs, two pairs of 

 which are directives, and usually a second series which arise antero- 

 posteriorly as isocnemic exoccelic pairs all round, becoming arranged 

 in cycles. Mesenterial filaments simple, without lateral ciliated 

 bands. Lower region of gastro-ccelomic cavity subdivided by septal 

 invaginations, alternating with the mesenteries, sometimes perforated 

 by skeletal growths. Reproduction sexual and asexual; asexual 

 reproduction frequent, by gemmation and fissiparity. 



