24 SIDERASTREA RADIANS. 



The absence of a siphonogl3'ph in Madreporaria affords an interesting con- 

 trast with the one or two grooves usually present in actinian and alcyonarian 

 polj^ps. A siphonoglyph is generally wanting only in the lowest actinians 

 and in alcyonarians, and its absence in coral polyps would suggest their 

 more primitive nature. This is further borne out by the absence of ciliated 

 bands or Flimmerstreifen from tlie mesenterial filaments (p. 29). 



The stomodaeal ectoderm is thicker than that of the disc, and differs 

 much from it in structure. Large nematocysts are distributed throughout, 

 but the main constituents are long, narrow, ciliated supporting cells, the 

 nuclei of which are arranged in a broad zone. The ciliation, which is uni- 

 form all round, generally persists in preserved material. Clear mucous 

 gland cells are not numerous, while here and there a granular gland cell 

 stands out very prominently by reason of the deeply staining character of 

 its contents. A distinct nerve layer is recognizable next the mesoglcea, but 

 no muscular fibrils can be detected. The endoderm is similar to that lining 

 the upper part of the polypal cavity. 



At its lower opening into the polypal cavity the ectoderm of the stonio- 

 dseum is reflected up the inner or endodermal surface, and then extends 

 horizontally for a short distance along the free edge and each face of the 

 perfect mesenteries, becoming continuous with their mesenterial filaments. 



MESENTERIES. 



The number and arrangement of the mesenteries in mature polyps are 

 as follows : (a) Six pairs are united with the stomodseum, including two 

 pairs of directives attached at the opposite extremities ; {5) an alternating 

 cycle of six pairs, the members of which are free from the stomodaeum 

 throughout their length ; {c) a variable number of third-cycle mesenteries, 

 alternating with the two previous cycles, but very rarely, if ever, with the 

 whole cycle of twelve pairs developed (plate 6, fig. 34). Thus two cycles 

 of mesenteries are always developed, and a variable number belonging to a 

 third cycle. They may be represented by the formula 6, 6, x, where x 

 represents any number from i to 12. 



Within each intermesenterial space, whether an entocoele or an exocoele, 

 an invagination of the polypal wall occurs which separates adjacent mesen- 

 teries from one another. Except in the uppermost region these septal 

 invaginations extend centrall}^ further than the mesenteries, while in the 

 lower parts of the polyp they extend so far as to meet in the middle and fuse, 

 dividing the gastro-coelomic cavity into distinct chambers, each of which 

 contains a mesentery (plate 7, figs. 38, 39). 



