Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FOEM OF THE BODY OF ECHINODEEMA. 197 Fig. 97. Diagram of the body- form of au Ophmrid. o Mont h. s Body disc, r Arms. § 160. The arrangements in other Echinoderma are derived from form which obtains in the Astero'ida; and that in two divergent lines. In Loth, although in different ways, it leads to a greater centralisation of the organism. Along one line the arms are more largely- developed, and at the same time gradually lose their relations to the internal organs. Along the other the
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FOEM OF THE BODY OF ECHINODEEMA. 197 Fig. 97. Diagram of the body- form of au Ophmrid. o Mont h. s Body disc, r Arms. § 160. The arrangements in other Echinoderma are derived from form which obtains in the Astero'ida; and that in two divergent lines. In Loth, although in different ways, it leads to a greater centralisation of the organism. Along one line the arms are more largely- developed, and at the same time gradually lose their relations to the internal organs. Along the other the arms pass completely into the common body. The number of rays appears to be always five. The first arrangement is seen in Brisinga and the Ophiurida, where the body is separated into a discoid central portion (Fig. 97, s), and the projecting, but sharply sepa- rated, arms (v). The arms take but a small share in the formation of the body-cavity, which is almost exclusively limited to the body disc. The ambu- lacral groove is wanting in the Ophi- urida, but the ambulacra still extend along the arms. The arms of the Euryalida are greatly developed, being repeatedly divided in a dichotomous fashion. A shallow groove is continued into the divisions. The Crino'ida, which in earlier periods were widely distributed, and very rich in forms, but which are now represented by few genera, have lost the power of free locomotion, and passed into the fixed condition. In the division Brachiata, which includes the extant forms, a stalk, often of great size, jointed, and complicated by branchings and appended struc- tures (Fig. 98), is developed on the antambulacral portion of the cup-like body. This serves as an organ of attachment. The arms are not always limited to five, but there is often a larger number of them ; they are con- siderably developed, being divided, or provided with secondary appendages. These appendages, which are attached to the arms and have the form of feathery plates, are
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