. Outlines of zoology. Zoology. 240 ECHINODERMA. Diatoms, Protozoa, minute Crustaceans—are wafted down ciliated grooves on the arms to the central mouth, which is of course on the upturned surface. Some members of the class, Comatula, are infested by minute parasitic " worms " (Myzostomata) allied to Chaetopods, which form galls on the arms. A lost arm can be replaced, and even the visceral mass may be regenerated completely within a. Fig. 104.—Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea.—After Milnes Marshall. The section is interra
. Outlines of zoology. Zoology. 240 ECHINODERMA. Diatoms, Protozoa, minute Crustaceans—are wafted down ciliated grooves on the arms to the central mouth, which is of course on the upturned surface. Some members of the class, Comatula, are infested by minute parasitic " worms " (Myzostomata) allied to Chaetopods, which form galls on the arms. A lost arm can be replaced, and even the visceral mass may be regenerated completely within a. Fig. 104.—Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea.—After Milnes Marshall. The section is interradial on the left, radial on the right, t., Cili- ated openings in body-wall; h., subepithelial ambulacral nerve ; /., water vascular canal; tentacle ' mouth ; s., intestine ; g., central plexus, with "chambered organ "at its base ; f., cffilom; R.^-RV., radial plates ; Br., brachial plates ; w.,muscle ; a., axial nerve-cord ; d., central capsule ; CD., centro-dorsal plate ; p., cirri; e., nerve branches from central capsule to cirri. few weeks after it has been lost. It has been suggested that the occasional expulsion of the visceral sac frees the Crinoid from parasites (Dendy). The animal consists of (1) a cup or calyx, (2) an oral disc forming the lid of this cup, (3) the radiating "arms," and (4) the stalk supporting the whole. The oral disc, turned upwards, is supported by plates. Heie the anus also is situated. The arms usually branch in dichotomous fashion, and thus ten, twenty, or more may arise from the original five. But the growing point continues to fork dichotomously, like the leaf of many ferns, and as each alternate fork remains short, a double series of lateral " pinnules " results. The arms are supported by calcareous plates. The stalk usually consists of numerous joints, especially in extinct forms, in some of which it measured over fifty feet in length. Except in Holopus, and in the stalked stage of Antedon, the stal
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
