A treatise on zoology . ex side; at the columnal endare two marginals, at the oralend are three, and none of thesefive continue on to the convexside, although correspondingplates occur there. The medianadoral marginal of the convexside (Fig. XIII. 2) is the plateM; its free edge is occasionallydenticulate (cf. ridgings in Mitro-cystis). The somatic plates ofthe concave side are one largecentral, and one small at itsleft upper corner : those of theconvex side are eleven, viz. two ad-colunmal, as in Mitrocystis, supportingone median which does not touch the column as it does in Enoijloura;a tran
A treatise on zoology . ex side; at the columnal endare two marginals, at the oralend are three, and none of thesefive continue on to the convexside, although correspondingplates occur there. The medianadoral marginal of the convexside (Fig. XIII. 2) is the plateM; its free edge is occasionallydenticulate (cf. ridgings in Mitro-cystis). The somatic plates ofthe concave side are one largecentral, and one small at itsleft upper corner : those of theconvex side are eleven, viz. two ad-colunmal, as in Mitrocystis, supportingone median which does not touch the column as it does in Enoijloura;a transverse row of five, the median of which is small and often quitesurrounded by its two neighbours (it is not an anal structure, as supposed) ;a row of three adjoining the adoral marginals. Stem much as in Mitro-cystis. The three marginals that meet at each adoral angle of the theca(Fig. XIII. 3) form an articular surface {Br) for the support of a spine ^ This name must be restricted to Wetherbys Fig. 1, d, e, f, and Fig. XIII. Placocystis Forhesianus. 1, concave or supposedright side; 2, convex or supposed left side ; 3, uppermargin, sliowing attachment for spine at Br.(1, reconstructed from various specimens in BritisliMuseum ; 2, from Brit. Mns., E7545, enlarged ; 3,from the type-specimen, Brit. Mus., E75SS.) THE CYSTIDEA (Br) which may attain 2/3 the length of the theca, without signs of ajoint. The spine is subcircular in section, and has no groove and noaccessory plates ; none the less it may have served as an arm, as thebearer of a tentaculate extension of the water-system, and of a ciliatedpath to the mouth. The mouth, anus, and hydropore were probablysituated in the integument uniting the two sides of the thecal openingthat seems to stretch between the spines. Haeckel (1896) supposes thatthe Apus4oTm of the Anomalocystidae was correlated with locomotion,and that the stem was a locomotor organ—a suggestion by no means far-fetched ; but his statement that the
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
