694 



RADIATA. 



Clypeaster and Scutella we have an approach towards the Asteriadce ; the shell being more or less flattened, and 

 divided at its margin so as to resemble the body of a Star-fish ; whilst the anus leaves its central position on the 

 upper surface, and approaches the mouth, which still retains its central position below. In the Spatangus and 

 its allies, the radiated form 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 to a bi-lateral symmetry ; and in this and some other particulars, the Spatangace® may be considered as 

 ieading towards the next group. 



V. The order Hoi.OTHURlAS.fi corresponds with Cuvier's Third Family of PediccUata (p. 641). 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, which 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 contraction of the in- 

 teguments in the manner of a worm ; so 

 that they may be designated Cihrhi-Vehmi- 

 guada. The cirrhi are not always developed 

 equally on all sides of the body, being some- 

 times confined to one side on which the 



Fig. 3 — Holoththia. animal creeps ; so that in this position it 



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

 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 respiratory appa- 

 ratus is regenerated. 



VI. In the ordtr Sipcscclid.e, which corresponds with Cuvier's Second Order Apoda, 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 entirely accomplished, like that of worms, by the contraction of their inte- 

 guments ; whence they may be designated Vekmigrada. In the general structure of their internal organs, how- 

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

 be properly regarded as belonging to the class Echiuodermata, which it links with the Articulate series. 



The recent discovery of an entirely new series of forms of Echinodermata, which abounded in the early ages of 

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

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

 mediate between the Crinoidece, the Echinidce, the Asteriadce, and the Ophiuridce ; for it combines within itself, 

 in a most remarkable manner, some of the distinctive characters of each of these groups. '* The Cystidea? 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 with 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, where 

 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 perforation, that of the base, is con- 

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

 Cystidecc. 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 which the name of pectinated rhombs may be 

 appropriately given." [Professor E. Forbes on the British Cystidecc, in the Memoirs of the Geological Survey of 

 Great Britain, Vol. II.] Thus in the attachment of the body by a stem, the Cystidecc resemble the Crinoidecc ; 

 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 Echinidce. 

 In the division of the body of certain genera into lobes, the approach the Asteriadce; and the arms, where they 

 are present, are more nearly allied, as regards their structure and origin, to those of the Ophiuridce than to those 

 of the Crinoidece. 



The Cyst idece and the Crinoidece seem to have been abundant in the earliest age of organic life on the earth, 

 namely, the Palccozoic period ; whilst the remains of the more highly organized Star-fish and Echini are but rarely 

 found in the rocks of that series. During the Secondary period, on the other hand, we find the latter 

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

 higher, and these in their turn all but disappear ; whilst the Cystideae seem to have become altogether extinct 

 »<3ry early in that series. In the Tertiary period we find a close approximation to the existing distribution of 

 Echinodermata. 



