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ANTHOZOA

pair within the seventh, and so on (Fig. 13). In the equina, the Edwardsia stage is arrived at somewhat differently. eighth Cerianthidea, as in the Zoanthidea, much as the adult arrangement The mesenteries second in order of formation form the sulcular of mesenteries differs from that of Actinia, the derivation from an directives, those fourth in order of formation form with the hit Edwardsia stock is obvious. the sulculo-lateral couples of the adult.

Fig. 11.—A, Diagram showing the sequence of mesenterial development in an Actinian. B, Diagrammatic transverse section of Gonactmia prolifera. As far as the anatomy of the zooid is concerned the majority of the stony or madreporarian corals agree exactly with thesoit-bodiea Actinians, such as Actinia equina, both in the number and anangement of the adult mesenteries and in the order of development ol the first cycle. The few exceptions will be dealt with later, but it may be stated here that even in these the first cycle of six couples of mesenteries is always formed, and in all the cases which have been examined the course of development described above is followed. There are, however, several groups of Zoanthana m which the mesenterial arrangement of the adult differs widely iiopi that just described. But it is possible to refer all these cases with more or less certainty to the Edwardsian type. , ,., The order Zoanthidea comprises a number of soft - bodied Zoantharians generally encrusted with sand. Externally they resemble ordinary sea-anemones, but there is only one ciliated groove, the sulcus, in the stomodseum, and the mesenteries are arranged on a peculiar pattern. The first twelve mesenteries are disposed in couples, and do not differ from those of Actinia except in size. The mesenterial pairs I, II, and III are attached to the stomodseum, and are called macromesenteries (Eig. 12, B), but I ,

Fig. 12.—A, Zoanthid colony, showing the expanded zooids. B; Diagram showing the arrangement of mesenteries in a young Zoanthid. C, Diagram showing the arrangement of mesenteries in an adult Zoanthid. 1, 2, 3, 4. Edwardsian mesenteries. V, and VI are much shorter, and are called micromesenteries, The subsequent development is peculiar to the group. New mesen teries are formed only in the sulco-lateral exocceles. They are formed in couples, each couple consisting of a macromesentery and a micromesentery, disposed so that the former is nearest to the sulcar directives. The derivation of the Zoanthidea from an Edwardsia form is sufficiently obvious. The order Cerianthidea comprises a few soft-bodied Zoantharians with rounded aboral extremities pierced by pores. They have two circlets of tentacles, a labial and a marginal, and there is only one ciliated groove in the stomodseum, which appears to be the sulculus. The mesenteries are numerous, and the longitudinal muscles, though distinguishable, are so feebly developed that there are no muscle - banners. The larval forms of the type genus Cerianthus float freely in the sea, and were once considered to belong to a separate genus, Arachnactis. In this larva four pairs of mesenteries having the typical Edwardsian arrangement are developed, in the same sequence as in Rhodactis, but the fifth and sixth pairs, instead of forming couples with the first and second, arise in the sulcar chamber, the fifth pair inside the fourth, and the sixth pair inside the fifth. New mesenteries are continually added in the sulcar chamber, the seventh pair within the sixth, the

Fig. 13.—A, Cerianthus solitarius (after A. Andres). B, Transverse section of the stoniodseum, showing the sulculus, si, and the arrangement of the mesenteries. C, Oral aspect of Arachnactis brachiolata, the larva of Cerianthus, with seven tentacles. D, Transverse section of an older larva. The numerals indicate the order of development of the mesenteries. The order Antipathidea is a well-defined group whose affinities are more obscure. The type form, Antipathes dichotomy (Fig. 14), forms arborescent colonies consisting of numerous zooids arranged in a single series along one surface of a branched horny axis. Each zooid has six tentacles ; the stomodseum is elongate, hut the sulcus and sulculus are very feebly represented. There are ten mesenteries in which the musculature is so little developed as to be almost indistinguishable. The sulcar and sulcular pairs of mesenteries are short, the sulco-lateral and sulculo-lateral pairs are a little longer, hut the two transverse are very large and are the only mesenteries which bear gonads. As the development of the Antipathidea is unknown, it is impossible to say what is the

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pIG -14 a Portion of a colony of Antipcithes dichotoma. B, Single zooid and axis of the same magnified, m, mouth; mf, mesenterial filament; ax, axis. C, Tranverse section through the oral cone of Antipathella, minor, st, stomodseum ; ov, ovary. sequence of the mesenterial development, but in Leiopathes glaberrima, a genus with twelve mesenteries, there are distinct indications of an Edwardsia stage. . There are, in addition to these groups, several genera of Actinians whose mesenterial arrangement differs from the normal type. Of these perhaps the most interesting is Gonactinia prolifera (Fig. 11 B), with eight macromesenteries arranged on the Edwardsian plan ’ Two pairs of micromesenteries form couples with the first and second Edwardsian pairs, and in addition there is a couple of micromesenteries in each of the sulculo-lateral exocceles. Only the first and second pairs of Edwardsian macromesenteries are fertile, t.e., bear gonads.