694 KADIATA. 



ClypeasUr and Scutdlx we have an approach towards the AsiermdrE ; the shell bting more or less flattened, and 

 divided at its mart^in fco as to resemble tlic body of a Star-fish ; whilst the anue leaves its central position on the 

 upper surface, and approaches the mouth, which still retains its central position below. In the &patm<giKS and 

 its allies, the radiated furm is considerably departed from ; the shell being oval instead of globular, and the 

 mouth and anus being neither of them central. In fact the radiated arrangement shows a tendency to give 

 place lo a bi-lateral symmetry ; and in this and some other particulars, the Spatangaces may be considered as 

 leading towards tlie next group. 



V. The order lioLuTnuniAD.TL corresponds with Cuvier's Third Family of Pedkellito. (p. €41). They combine, in 



a very curious manner, the radiated ar- 

 rangement of the surface and oral appen- 

 dages, which is characteristic of the Echino- 

 dermata, with the bi-lateral symmetry of 

 the internal organs, wliich is characteristic 

 of Articulated animals. Many of them, 

 moreover, exhibit indications of an obscure 

 transverse division of the soft body into 

 segments, as in the Annelida. The movement 

 of the body is partly effected by the cirrhi, 

 and in part by the eontraetion of the in- 

 teguments in the manner of a worm ; so 

 tliat they may be designated Cinnni-VEiiMi- 

 GRADA. Thecirrhi are not always developed 

 equally on all sides of the body, being some- 

 limes confined to one side un which the 

 Fii-. 3-— HoLoTHUKiA. animal creeps ; so that in this position it 



presents, as it were, a back and a belly. It is a very extraordinary fact in regard to these animals, that, \vlieu 

 they are irritated, the whole of the viscera are frequently discharged from the interior, the body remaining as 

 an empty sac ; and yet that, after a time, the whole of the complex digestive, circulating, and resjjiratory appa- 

 ratus is regenerated. 



VI. In the ordtr SiPUNCULiD.E, which eorresponds with CuA-ier's Second Order .-Ij'OJa, the radiated arrange- 

 ment still more completely gives place to the annular. In their external appearance they are worms ; they have 

 no cirrhi ; and their progression is cntii'tly accomplished, like that of worms, by the contraction of their inte- 

 guments ; whence they may be designated Vebmigrada. In the general structure of their internal organs. ho\v- 

 ever they bear a much closer resemblance to the Ilolothuriadce than to Annulose animals, and must therefore 

 be properly regarded as belonging to the class Echinodermata, which it links ^\ith the Articulate series. 



The recent discovery of an entirely new series of forms of Eehinodermata, wliieh abounded in the early ages of 

 the earth's history, but wliich seems to have become entirely extinct before the Pentacrinites were called into 

 existence, has rendered it necessary to institute a new order, the Ctstideje, the place of which seems to be inter- 

 mediate between the C'raioi^eo?, the i!.'6'Ai/ii(i'.r, the Astcfiad-x, a.nd the Ojyhiuridai ; for it combines within itself, 

 in a most remarkable manner, some of the distinctive characters of each of these groups. " The Cystide:E are 

 more or less spherical bodies covered with polygonal plates, varying in number according to the genus, closely 

 fitting together so as to invest the entire surface with a compact coat of mail, except at four points, viz. in- 

 feriorly, where the body unites ■\vith a stem ; centrally, or above the centre on one side, where there is an opening 

 closed by valves, supposed with good reason to be the orifice of the reproductive system ; and superiorly, wliere 

 the mouth is found, usually if not always with a small perforation, supposed to be a vent, alongside of it. These 

 parts, viz. the plates investing the body, the three orifices (for the fourth perfoi-ation. that of the base, i? con- 

 tinuous with the canal of the stem where the latter is well developed), and probably the stem, are common to all 

 Cystidea\ There are other parts, apparently of great consequence in the organization of the animal, which are 

 common only to certain members of the order. These are the brachial appendages (arms and tentacula) and 

 certain curious organs or appendages connected with the plates, to wliich the name of pectinated rliomhti may be 

 appropriiitely given." [Professor E. Forbes on the British Cysiidea-, in the Memoirs of the Geological Survey of 

 Great Britain, Vol. II.] Thus in the attachment of the body by a stem, the Cystldcai resemble the Crino'u.Uur ; 

 and some of the aberrant forms of these two orders come into very close approximation with each other. In the 

 complete enclosure of the body within a shell composed of polygonal plates, they correspond with the Ecldnidtc. 

 In the division of the body of certain genera into lobes, the approach the Jp(r;ria'i'.T; and the arms, T\here they 

 are present, are more nearly allied, as regards their structure and origiu, to those of the C>2.'hii'rid:v than to those 

 of the Criiioidciir. 



The C^s(j(/ca.' and the CHvoi^efe seem to have been abundant in the earliest age of organic life on the earth, 

 namely, the PukfOzoic period ; whilst the remains of the more highly organized Star-Jisk and Echini are hut rarely 

 fumid in the rocks of that series. Dmiug the Secaudary period, ou the other hand, we find the latter 

 gradually becoming more numerous, and their forms more varied ; the lower funns of Crinoidex give place to the 

 higher, and tlirse in their turn all but disappear; whilst the Cystidere seem to have become altugetJu-r extinct 

 'ery early in that series. In the Tertiary period we hud a close aiiproximatioa to tiie existing distribution of 

 Eehinodermata. 



