. The Cambridge natural history. Zoology. ANATOMY OF OPHIOTHRIX 479 jaw. One of the commonest British Ophiuroids is Ophiothrix fragilis (Figs. 207, 208), which is found in swarms in shallow water off the west coast of England and Scotland. We may therefore select it as the type, and, since the arm is the most characteristic organ of an Ophiuroid, we may commence by study- ing it. Speaking generally, an Ophiuroid either drags itself forward by two arms and pushes itself by the other three (Fig. 207),^ or else it drags itself by one and pushes with the other four (Fig. 217). The arms during
. The Cambridge natural history. Zoology. ANATOMY OF OPHIOTHRIX 479 jaw. One of the commonest British Ophiuroids is Ophiothrix fragilis (Figs. 207, 208), which is found in swarms in shallow water off the west coast of England and Scotland. We may therefore select it as the type, and, since the arm is the most characteristic organ of an Ophiuroid, we may commence by study- ing it. Speaking generally, an Ophiuroid either drags itself forward by two arms and pushes itself by the other three (Fig. 207),^ or else it drags itself by one and pushes with the other four (Fig. 217). The arms during this process are bent into characteristic curves, by the straightening of which in the posterior arms the animal is pushed onwards, whilst the intensification of these curves in the anterior arms causes the animal to be dragged forwards. The grip of the arm on the substratum is chiefly in the distal portion P°^- of the curve. The alteration of the curvature is due to the ' contraction of the muscles on one side of the armq l^hprp ^'°- 208.âOral view of the disc of OjMothHx one sme 01 tne arms. mere fragilis. (/.J, Opening of the genital bursa; is no ambulacral groove such «>-P, madreporlte ; pod, podia ; , tooth- . i. 1 ,1 -I â ^ papillae; , ventral plates of the arms. as IS lound on the under side ^ i. of the arms of all Asteroidea, - for the arm is completely ensheathed by four series of plates, an upper row of dorsal plates, an under row of ventral plates, and two lateral rows of lateral plates. The last named, which in cill probability correspond to the adambulacral plates of Starfish, bear each a transverse row of seven spines with roughened surfaces; these enable the animal to get a grip on the substratum over which it moves. The podia in Ophiuroidea are termed " tentacles"; they are totally devoid of suckers, being simple conical papillae used as sense-organs, and are of little, if any, service in locomotion. They issue from openings cal
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