CHAPTER XXIII. 

 OF THE CLASS OF ECHINODERMATA. 



1090. THIS Class is unquestionably to be regarded as the 

 highest, in point of complexity of structure, among those which 

 constitute the Radiated division of the Animal Kingdom. It 

 is characterized by the very regular disposition of parts round a 

 common centre, which is found in nearly all the animals com- 

 posing it, and which is never departed from in any remarkable 

 degree ; but chiefly by the hard integument, beset with spines 

 or prickles, from which its name is derived, and which may be 

 regarded as its chief external mark. This character, however, 

 is not presented by all the species which, on account of the 

 general similarity of their organisation, are associated in the 

 group ;* but it is decidedly absent only in those which evi- 

 dently constitute links of transition towards other divisions 

 of the animal kingdom. In no being is the radiated form more 

 distinctly marked, than in the Astenas or Star-fish (Fig. 708), 

 which is one of the best known species of this Class. And it 

 exists not only in its external aspect, but in the arrangement of 

 its internal organs, which are all disposed with perfect regularity 

 around a common centre. In the Echinus or Sea-Urchin 

 (Fig. 706) it is equally manifested in the arrangement of the 

 plates which form its nearly globular shell (commonly termed 

 sea-egg} ; but the more highly-organised condition of the diges- 

 tive apparatus prevents the radial symmetry from being equally 

 shown in the internal viscera. In the Holothurice and their allies 

 (Fig. 710), it is still preserved in the circular form of the body, 

 and in the disposition of the appendages around the mouth ; 

 although the globular shape of the Echinus gives place to a more 

 prolonged form, which, in some species, even approaches to that 

 of the Vermiform tribes (Fig. 711). 



* The tubular feet hereafter to be described ($ 1096) are more universally 

 present than the spines; hence it has been proposed to designate the Class 

 Cirrhodermata. 



