304 



PHYLUM ECHINODERMATA. 



we prefer to subordinate the Cystidea and Blastoidea to no 

 section of the Echinodermata and to treat them as independent 

 classes, which in respect of their upturned mouth, their orally 

 placed anus, and their aboral peduncle of attachment recall 

 Crinoids, but which in other features of their anatomy show 

 considerable diversity, some having obvious leanings towards 

 Echinoids and Asteroids, and some seeming to be representative 

 of a stage of structure in which the symmetry of the modern 

 Echinoderm had not been evolved. It is indeed highly probable 

 that the Cystids as at present constituted contain heterogeneous 

 elements, which should be assigned to more than one independent 

 class. Already highly competent authorities have separated 

 from them the Edrioasteroidea * and there is much to be said 

 for separating other members of the class. 



Class CYSTIDEA.t 



Stalked or unstalked forms with calyx which is usually composed of irregu- 

 larly arranged plates. Arms are imperfectly developed or absent, and the 

 radiate symmetry is often very imperfect or absent. The plates of the calyx 

 usually possess pores. 



The Cystidea are entirely extinct and only known to us by their fossil 

 remains, which so far as our present information goes are confined to the 

 Palaeozoic rocks. Forthis reason their structure is and must remain very 

 imperfectly known, and the class presents great difficulties to the zoolo- 

 gist. These cannot be overcome and we must content ourselves in this 



work with the description of a few forms 

 which seem to represent the considerable 

 range of structure found in the group. For 

 fuller information we refer the reader to the 

 excellent account contained in the two works 

 cited. 



The most significant characters of the 

 Cystids are (1) the irregularity in the arrange- 

 ment of the thecal plates which is so often 

 found in them ; (2) the absence or feeble 

 development of arm-like structures ; (3) the 

 fact that the generative organ seems usually 

 to have been single and not radially arranged ; 

 (4) the fact that in some of them there does 

 not appear to have been a radial symmetry. 

 If we are to have phylogenetic speculations 

 these characters must obviously be taken into 

 verv careful consideration. 



FIG. 211. Cystaster graniilatus 

 from the posterior side show- 

 ing the oral surface in perspec- 

 tive. The two left-hand rays 

 retain the covering plates 

 which are absent from the 

 others (from Bather) x 3. As 

 anus ; cp covering plates ; o 

 peristomial plates ; sp side 

 plates. 



* See Bather, op. cit. 



t Zittel's Text-Book of Palaeontology, vol. 1, London, MacMillan & Co., 

 1900. F. A. Bather, chapter on Pelmatozoa in Lankester's Treatise on 

 Zoology, London, A. and C. Black, 1900. O. Jaekel, Stammesgeschichte 

 der Pelmatozoen, Berlin, 1899. 



