ZOANTHARIA 187 



In its main structural lines the zoantharian polyp resembles the 

 alcyonarian type. The stomodaeum is elongated in the same plane but 

 possesses two siphonoglyphs instead of one. There are tentacles which 

 are hollow, unbranched, and often very numerous. The mesenteries 

 are like those of Alcyonium, but their arrangement and the structure 

 of the mesenteric filament is very different. Numbers and grouping 

 of mesenteries vary greatly within the limits of the Zoantharia itself. 

 The simplest form, and that most like Alcyonium (Fig. 140 A), is found 

 in the small burrowing sea anemone, Edwardsia (Fig. 140 C). Here 

 there are eight mesenteries with bilateral symmetry, as in Alcyonium. 

 In six of these the longitudinal muscles are on the same side, facing 

 ventrally, while the remaining pair have the muscles facing outwards 

 and dorsally, so that the arrangement is different from that in the 

 Alcyonaria. 



In the typical sea anemone, such as Actinia, and in coral polyps, 

 the mesenteries are arranged in cycles (or generations). There are six 

 couples of primary mesenteries in the first cycle, and these are the 

 largest and alone reach as far as the stomodaeum. In four of these 

 pairs the muscles face each other ; in the other two pairs, the directives, 

 they face away from each other. The secondary mesenteries, which 

 are much smaller, are situated in the spaces between two adjacent 

 pairs (exocoeles), never between two members of a pair (entocoeles). 

 Finally, there may be tertiary and even quaternary mesenteries, al- 

 ways in exocoelic spaces of the generation preceding, making third 

 and fourth cycles. This *' hexactinian" type, in which the mesenteries 

 are present in multiples of twelve, is derived from that in Edwardsia, 

 as may be seen in the development of some of the Zoantharia, for 

 example another small burrowing anemone, Halcampa. In this there 

 is first of all an Edwardsia stage (Fig. 140 C) with eight mesenteries. 

 From this the hexactinian type is derived quite simply by the sub- 

 sequent growth of four additional mesenteries with muscles on their 

 dorsal faces. These belong to the first cycle and join up with the 

 stomodaeum, and they arise in such positions as to complete, with 

 pre-existing mesenteries, four pairs with muscles facing each other. 

 These four mesenteries in Halcampa never develop a mesenteric 

 filament, but the complete adult arrangement, as seen for instance in 

 Actinia mesembryanthemum, the commonest of our British anemones, 

 is given in Fig. 140 E. In such a form as Peachia, often used in labora- 

 tories on account of its simplicity, there are slight deviations from the 

 type. There is no second siphonoglyph (sulculus) and the second cycle 

 of mesenteries is incomplete, none of them having a mesenteric fila- 

 ment, while the pairs in two exocoeles are completely absent (Fig. 

 140 B). 



The mesenteric filament of the Zoantharia (Fig. 141 B, C) is tre- 



