164 THE INVERTEBRATA 



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. 136 A), is found in the small burrowing sea anemone, Edwardsia 

 (Fig. 136 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, 

 always 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. 136 C) with eight mesen- 

 teries. From this the hexactinian type is derived quite simply by the 

 subsequent 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 seen 

 in Fig. 136 E. In such a form as Peachia, often used in laboratories 

 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 filament, 

 while the pairs in two exocoeles are completely absent (Fig. 136 B). 



The mesenteric filament of the Zoantharia (Fig. 137 B, C) is trefoil- 

 shaped in section, and while the functions of digestion and water- 



