THE CYSTIDEA 39 



value ; and to place them either in a line or side by side does not 

 represent their phylogenetic relations. Such, probably, would be 

 better shown by placing a primitive class, Amphoridea, at the 

 base and deducing from it several lines of descent, viz. Edrioas- 

 teroidea, Anomalocystida, Aporita, Rhombifera, and Diploporita. 

 From the Edrioasteroid line, we may suppose, there sprang first 

 Holothurians, then Stelleroidea, then Echinoidea, while the line 

 itself still survived in more specialised forms to the close of the 

 Carboniferous period. The Diploporite line ought properly to 

 include the Blastoidea ; and from it probably there arose, as a fresh 

 development with a new lease of life, the important class, 

 Crinoidea. The other lines were unsuccessful and none survived 

 the Silurian. But to make the classification coincide absolutely 

 with this history, which after all is not yet proven, would be to 

 reject names and classes that have held the field for more than 

 half a century in favour of new and unaccepted terms. Old 

 names, therefore, have been retained so far as possible. The 

 diversity of existing opinion, however, may serve as excuse for a 

 few novelties. Such are the use of Haeckel's Amphoridea, in an 

 emended sense ; the resuscitation of Edrioasteroidea ; the emenda- 

 tion of the Rhombifera, Aporita, and Diploporita, and of the 

 included families, which, when not new, are rarely used in the 

 sense of the original proposer ; the extension of the Blastoidea, 

 and their division into Proto- and Eu-blastoidea ; a considerable 

 revision of the accepted classification of Eublastoidea ; and a 

 recasting of the classification of Crinoidea. 



CLASS I. CYSTIDEA, VON BUCK (1844). 



Order 1. Amphoridea. 



2. Rhombifera. 



3. Aporita. 



4. Diploporita. 



Pelmatozoa in which radial polymeric symmetry of the theca 

 is developed either not at all or not in complete correlation with 

 the radial symmetry of the ambulacra (such as obtains in Blastoidea 

 and Crinoidea) ; in which extensions of the food -grooves are 

 exothecal or epithecal or both combined, but neither endothecal 

 nor pierced by podia (as in Edrioasteroidea). 



The earlier and more primitive Cystidea represent the pelma- 

 tozoic stage through which the Echinoderm race passed, on its 

 way from the Dipleurula to the various classes. They shed light 

 not only on the origin of those classes, but on the still more 

 ancient ancestor of the Phylum. The remarkable adaptability of 

 the Echinoderm type, the mode of origin of many organs, and the 



