. The annals and magazine of natural history : zoology, botany, and geology. mbersof the third order. Other polyps which have been examined at the bioralstage reveal tiiat the arrangement of the mesenteries is notalways so regular as that here given. Yet the essential factsare invariably the same, namely, the insertion of one pair ofdirectives and a certain number of pairs of the originalcomplete mesenteries on each stomodeeum and the entocoelicplane of separation of the two moieties. As the bioral polyp continues its growth more pairs ofmesenteries extend as far as each stomodseum, and other
. The annals and magazine of natural history : zoology, botany, and geology. mbersof the third order. Other polyps which have been examined at the bioralstage reveal tiiat the arrangement of the mesenteries is notalways so regular as that here given. Yet the essential factsare invariably the same, namely, the insertion of one pair ofdirectives and a certain number of pairs of the originalcomplete mesenteries on each stomodeeum and the entocoelicplane of separation of the two moieties. As the bioral polyp continues its growth more pairs ofmesenteries extend as far as each stomodseum, and other newpairs arise on the polypal wall in a more or less irregularmanner. The next fission-stage available is one in which four oralapertures are present on an elongated disk surrounded by asingle system of tentacles and a single column-wall. Stageswith three oral apertures seem to be very rare, so thatevidently fission of each of the two stomodaa occurs atabout the same time. Fig. 3 represents the essential rela-tionships of the mesenteries and the four stomoda^a. By Fig. Diagrammatic arrangement of the mesenteries in a polyp of Manicinaareolata with four stomodseal systems. this time scarcely any constancy is evident in the number ofmesenteries attached to each stomodfeura or in the number ofincomplete pairs which may occur in the exocoele betweenany two complete pairs. Only the two primary pairs of directives still occur, situated Morphology of the Madreporaria. 389 at the opposite extremities of the system, wliere growth isgenerally proceeding more rapidly than elsewhere. No newdirectives are found ever to arise. A variable number ofmesenterial pairs is associated with each stomodasum, thehexameral plan being wholly departed from. Except inone doubtful case, the plane of fission has been found to bealways included within the entocoele of two opposite pairs ofcomplete mesenteries. During the subsequent growth of the multioral polyp theoriginal linear arrangement is soon lost
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