326 



COMPARA TIVE ANA TOMY 



CHAP. 



Although there are good palseontological reasons for the generally accepted belief 

 that all known exocyclic (irregular) Echinoidea are descended from endocyclic 

 (regular) forms, it has been conjectured that these latter may themselves have had 

 exocyclic ancestors (which, indeed, are unknown to us). Thus the modern Sjiutnn- 

 cjoida and <J!iij>mstroida, for example, by the position of the anus in the posterior 

 unpaired interradius, may secondarily have attained a primitive condition. The 

 anus would then have wandered first from the posterior unpaired interradius to the 

 centre of the apical area, and then, in the exocyclic forms known to us. have shifted 

 back again in the same direction. This suggestion, which is of special significance 

 with reference to the primitive Pcfmatozoa, receives some (not vei-y satisfactory) 

 support from the fact that in the very old family of the Sn/cniidcc among the regular 

 Echinoidea, the anus lies at the posterior edge of the apical system in the oldest 

 forms, but during geological development approaches more and more near the centre 

 of the system, near which it is found asymmetrically (posteriorly to the right) in 

 the modern forms. 



II. Asteroidea. 



The typical plates of the apical system are not present in most 

 adult Star-fish, or at any rate cannot be made out among the numerous 

 calcareous pieces embedded in the dorsal area of the disc. There are, 



however, exceptions to this rule. For 

 instance, in species of the genera Pinfn- 

 gonaster, Tosia, Astrogonium, Stellaster, 

 Ned rut, Ferdina, Pentaceros, Gymnasteria, 

 Sajtastcr, Ophidiaster, Zoroaster, the central 

 plate, the five basals and the five radials 

 can still be more or less clearly recognised 

 in the adults. Occasionally (in species 

 of Pentagonaster, Ghjmnasteria, Pentaceros, 

 and many Goniasteriike) there are even to 

 be found plates which in position corre- 

 spond with the infrabasals. The whole 

 apical system is specially well developed 

 in young specimens of the deep-sea Star- 

 fish Zon nister fulgcns (Fig. 286). The 

 aperture of the stone canal lies in the 



in "young 7pecTmen" oTVoroaster right anterior interradius, outside the 

 fuigens (after siaden). For lettering basal ; the anus in the right posterior 



interradius, inside the basal. In all 



Asteroids, the madreporic plate and anus lie in these interradii of the 

 apical region (cf. the Echinoidea, Figs. 272-275). 



The typical apical system can also be proved ontogenetically in 

 Star-fishes, even in forms in which it is absent or unrecognisable in 

 the adult. Five basals, a central plate and five radials are actually 

 among the first plates formed in the embryo Star-fish, in the very 

 order in which they are here named, though always after the terminals, 

 presently to be described, which appear first of all. Small plates, 

 appearing radially within the circle of basals, have been considered to 



FIG. 2S<3. Apical system of plates 



