14 ECHINODERMA GENERAL DESCRIPTION 



Echinoderma. The orals on the one side are supposed by some 

 (e.g. Loven, P. H. Carpenter, Sladen) to be represented by some- 

 what similarly situated plates in Stelleroidea ("buccal shields" of 

 Ophiuroids, " odontophores " of Asteroids), and by a circumoral 

 calcareous ring in Holothurians. The plates of the dorsal cup 

 have been homologised with plates very similar in shape and 

 arrangement that are often to be observed in Stelleroidea, notably 

 on the aboral side of the compact body of Ophiuroids, arid with two 

 circlets of plates at the aboral pole of Echinoids. It has been 

 supposed that all Echinoderma primitively possessed a definite 

 calycinal system, thus composed : a central aboral plate (" dorso- 

 central"), five plates surrounding this ("basals," "genitals" of 

 Echinoids), five plates following on these and alternating with 

 them ("radials," "terminals" of Asteroids, "oculars " of Echinoids) ; 

 these together formed the "apical system," and to them was some- 

 times added a circlet below, and alternating with, the basals (" infra- 

 basals ") : five orals, alternated with the radials, and to these P. 

 H. Carpenter once added an " oro-central," the correlative of the 

 dorso-central. The oro-central is a discredited myth. The dorso- 

 central is a plate at the distal end of the Crinoid stem, i.e. in the 

 preoral lobe (fp in Fig. X.) ; there is no proof that it ever formed 

 part of an apical system, and it cannot be considered either homo- 

 genetic or'homoplastic with the aboral central plate sometimes seen 

 in Stelleroidea. As for the basals and radials 

 of the Crinoid, they are, as stated above, 

 formed around the right posterior coelom ; 

 this also is the position of their supposed 

 homologues in Asterina (Fig. XII.), and 

 MacBride's argument that their relations 

 to the stem are different, does not seem 

 fatal to the above theory. What is fatal 

 is the conclusion to which the evidence of 

 FIG XII fossils forces us that the free Echinoderms, 



Dorsal, i.e. 'aboral, view of if tne j arose from Stalked forms at all, 



Asterina gibbosa, ten days old indubitably did so ages before a calycinal 



(ciiti6r ijiHlwigj. follows prGor&l " * 



lobe (p.i) now on oral side ; system had been evolved. Even among 



rudiments of five arms marked 1 n i r -i. A i , i 



by five terminal plates (r);ai- stalked torms it appears that regular apical 

 bSs'c/o^'and^atVis the s y stems arose independently in different 

 so-caiied dorso-centrai; jir, lines of descent. If, however, it be im- 



madreporite. x56. ... , ' . ' 



possible to regard the apical systems of 



Echinoidea and Stelleroidea as homogenetic with that of Crinoidea, 

 there can be no objection to the statement that similar plates are 

 developed in a similar position with regard to the fundamental 

 anatomy, under the influence of somewhat similar causes. 



The Asteroids were probably the last group to branch off from 

 the fixed Echinoderms. Hence it is that they retain many features 



