ix PHYLUM ECHINODERMATA 409 



produced by the action of their cilia serve to keep constantly 

 renewed the water in the neighbourhood of the anus and of the 

 respiratory podia or the papulae. 



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 central plate ; (2) of 

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

 which are radial in position. In the Asteroidea (Fig. 324) 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 are 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 arrangement resembles 

 that observable in the Asteroidea. In the Echinoidea (Fig. 328) 

 the basals (genitals) are perforated by the ducts of the repro- 

 ductive organs ; the radials (oculars) are perforated for the tentacles : 

 the central (anal) rarely persists as a single plate in the adult, 

 usually becoming broken up into a number of irregular plates. In 

 the stalked Crinoidea the term 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, uniting with the central 

 and the infra-basals, is transformed into a centro-dorsal plate, and 

 the basals nearly always unite into a rosette-plate, which is concealed 

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

 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 proceeds 

 a system of radially disposed arms or rays. The central disc and 

 the rays are usually compressed in the vertical direction, 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 number, except in malformed individuals ; but in 

 some they are six, in others seven, eight, or more. The proportions 

 borne by the arms to the central disc are subject to considerable 

 variation. In some, as in Asterias, the arms are long, and the 

 central disc appears as little more than their point of union ; in 

 others, again, owing to coalescence of the arms, the whole Starfish 

 has the form of a five-sided disc, in which the arms are represented 



