240 DESCRIPTIONS OF PREPARATIONS. 



others by a greater, &c. : hence they are often termed secondary, tertiary, 

 &c. The mesentery exposed in the upper part of the preparation is com- 

 plete : the one reflected in the lower part is incomplete. As the greater 

 portion of the substance of the wall has been removed, the external edges 

 of the mesenteries are clearly visible and it may be seen how great is their 

 number. They are really grouped in pairs, and the space between the two 

 members of a pair is known as the intra-septal, that between two adjacent 

 pairs as the inter-septal, chamber. The pairs are also so grouped that between 

 two primary pairs there is one secondary pair ; between a primary and a 

 secondary pair one tertiary, and so on. 



If the complete mesentery in the upper part of the preparation is care- 

 fully examined it will be seen to have two perforations. One of these, the 

 inner septal stoma, lies about J inch from the circumoral sphincter and is 

 found universally among Sea-Anemones piercing the primary or complete 

 mesenteries. The other perforation or outer septal stoma occurs in very few 

 instances. It pierces all the mesenteries and lies just within the little curve 

 made by the margin external to the outermost tentacle. Its presence indi- 

 cates the existence of a very well developed marginal sphincter, ' Rotteken's 

 ring-muscle/ which contracts the margin over the tentacles when the peri- 

 stome is retracted, but it is, however, not visible in this preparation. 



The incomplete mesentery which has been reflected has a thickened 

 inner free margin. This thickening is the mesenterial filament or craspedon ; 

 it is slightly convoluted. The free edges of all the mesenteries, complete 

 and incomplete alike, are similarly bordered ; and the convolutions of the 

 filaments are very visible on the inner surface of the coelenteric space or 

 stomach below the free edge of the stomodaeum. 



All Anthozoa possess a wall, a peristome with marginal tentacles, a 

 stomodaeum, and a coelenteric space subdivided by mesenteries. The 

 Actiniaria or Malacodermata possess simple non-pinnate tentacles ; the 

 number of their mesenteries is usually some multiple of the number six ; 

 and there is no hard skeleton. But some corals, e.g. Caryophyllia cyathus 

 and Madrepora variabilis (Koch, M. J. v. 1880), possess the same paired 

 mesenterial arrangement as do the Actiniae. 



The oesophagus or stomodaeum is formed as an invagination of the oral disc, 

 and is consequently lined by ectoderm. In the development of a Hexactinian 

 (in Hertwig's sense) twelve mesenteries appear, which group themselves in pairs. 

 These are, properly speaking, the primary mesenteries. The remaining mesenteries 

 of the adult appear in pairs in the primary inter-septal chambers. A certain 

 number of these mesenteries which are really secondary generally fuse with the 

 stomodoeum, thus becoming as it were primary. In Sagartia the really primary 

 mesenteries are alone thus connected. 



If the surface of a mesentery be carefully examined, and it is always with 



