418 ZOOLOGY 



SKCT. 



which are radial in position. In the Asteroidea (Fig. 319) 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. 323) 

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

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

 the 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 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 rossette-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 proceed 

 a series 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 propor- 

 tions borne by the arms to the central disc are subject to consider- 

 able 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 only by the five angles ; while between these two 

 extremes there are numerous intermediate gradations. The 

 Brisingidce differ from all the rest of the class in having the arms 

 almost as sharply separated off from the central disc as in the 

 Ophiuroids. 



The abaclinal or aboral, and the actinal or oral surfaces are always 

 distinctly marked off from one another. In the middle of the 

 latter (Fig. 335) is the mouth, running out from which are five or 

 more narrow ambulacral grooves, one of which is continued along 



