Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 APPENDAGES OE ECHINODERMA. 199 functions as ventral, are retained in Psolus, while the other two which belong to the portion of the surface of the body, which func- tions as the dorsal, are atrophied. In Cuvieria this modification of parts is extended so as to result in the disappearance of the three ventral ambulacra. A complete degeneration of the ambulacra takes place in the Synaptfe, and the radiate organisation, which is implied by the division into ambulacra, is thus lost; they have
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 APPENDAGES OE ECHINODERMA. 199 functions as ventral, are retained in Psolus, while the other two which belong to the portion of the surface of the body, which func- tions as the dorsal, are atrophied. In Cuvieria this modification of parts is extended so as to result in the disappearance of the three ventral ambulacra. A complete degeneration of the ambulacra takes place in the Synaptfe, and the radiate organisation, which is implied by the division into ambulacra, is thus lost; they have followed out the line of modification indicated in those Asteroida, where the rays begin to lose the organs belonging to them, as if in preparation for a centralised organisation. Appendages. § i62. The organs appended to the integument, which may be regarded as appendages, are not so various as in the Vermes. Of such parts the sucking or ambulacral feet must be placed foremost, for they form the most common arrangement, and belong to the typical Echinoderm organisation, and are evidently derived from an ancestor common to the whole group. They are tubular, and generally cylindrical processes of the body-wall, which not only agree in being arranged in rows (in accordance with the metamerism of the rays), but also in their most essential points of structure, with the parapodia of the Annelides : on the whole they are more simple in character. Fig. 100. Diagram of the cross section of an arm, A, of Asteracanthion rnbens; B, of Opliiura texturata. -p Ambulacral feet, p' AmpiTllfB. t Dermal tentacles. tv Nerve-chords, v) Ambulacral plates, in Muscles, a Ambulacral vein, h Ventral plate, c Lateral plates, cl Dorsal plate. 7; Calcified portion of the integnment (after Wilh. Lange). (Fig. 100). As they have always much the same structure they vary but little in function. The free end of these tubular structures (p) is either flattened out, or provided with a sucker-like terminatio
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