384 ZOOLOG/ SECT, 



clavulse in being covered with strong cilia, occur also in the 

 Clypeastroids and some Asteroids. The currents produced by the 

 action of their cilia serve to keep constantly renewed the water 

 in the neighbourhood of the anus and of the branchiae. 



There are two principal systems of plates to be recognised,. 

 an oral and an apical ; the former corresponding with the oral or 

 actinal* and the latter with the aboral or abactinal surface. The 

 former vary considerably in the different classes : the constant 

 elements are five orals, which may or may not be recognisable in 

 the adult. The apical system consists (1) of a dorso-central plate - r 

 (2) of five basals which are inter-radial in position; (3) of five 

 radials, which are radial in position. In the Asteroidea (Fig. 292) 

 the radials are late in making their appearance ; before they are- 

 developed five terminal plates have become distinct, one at the 

 end of each rudimentary arm ; these become carried outwards by 

 the extension of the arm, and each supports the corresponding 

 tentacle. As a rule these plates of the apical system are only 

 distinct in the young condition. In the Ophiuroidea the arrange- 

 ment resembles that observable in the Asteroidea. In the Echi- 

 noidea (Fig. 295) the basals (genitals) are perforated by the ducts 

 of the reproductive glands ; the radials (oculars) are perforated for 

 the eye and tentacle : the dorso-central (anal) rarely persists as a. 

 single plate in the adult, usually becoming broken up into a series 

 of irregular plates. In the stalked Crinoidea the term dorso- 

 central has been applied to a plate which is transformed into the 

 disc of attachment at the base of the stalk, but the correspondence 

 between this and the similarly named plate in the other classes 

 is very doubtful; the ossicles of the stalk intervene between, 

 it and the basals. In the free forms the uppermost segment 

 of the larval stalk is transformed into a centro-dorsal plate, and th 

 basals nearly always unite into a rosette plate, which is conceale 

 from view by the centro-dorsal and the radials. The apical syste 

 of plates is apparently not represented in the Holothuroidea. 



Modifications of Form in the Five Classes. The general 

 shape in the Asteroidea is, as already pointed out, that of a star.. 

 There is a central part, or central disc, from which proceed a series 

 of radially disposed arms or rays. The central disc and the rays 

 are usually compressed dorso-ventrally, as in Anthenea and 

 Asterina, but in some Starfishes the rays are approximately 

 cylindrical ; they nearly always taper distally. In the majority of 

 Starfishes, as in the examples described, the arms are five in nam 

 ber, except in malformed individuals ; but in some they are six, i 

 others seven, eight, or more. The proportions borne by the arm 

 to the central disc are subject to considerable variation. In som 

 as in Asterias, the arms are long and the central disc appears 

 little more than their point of union ; in others, again, the whole 

 Starfish has the form of a five-sided disc, in which the arms arc- 



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