452 CLASSIFICATION OF RADIATA. 



from the body. Its duration then varies according to the species 

 in which it is observed ; and it is influenced by many external 

 circumstances. It has been seen fifteen days after death in the 

 Tortoise, when putrefaction was far advanced ; and in the River- 

 Mussel it seems to endure with similar pertinacity. 



1089. The Classes which are referred to this Sub-Kingdom, 

 are the following : 



I. ECHINODERMATA, or prickle-skinned animals. This Class, 

 which includes the Echinus (sea-egg or sea-urchin), the Asterias 

 (star-fish), and many less known forms, displays the radiated 

 structure in its most striking form. The bodies of these animals 

 are covered with a firm integument, which is thickly beset with 

 spines ; and from this, the name of the Class is derived. In 

 many species this integument consists of calcareous plates, regu- 

 larly jointed together ; and in all it possesses greater toughness 

 than we elsewhere meet with in the Radiated Sub-Kingdom. 

 This is a character by which they are easily recognised, and are 

 well known. There are, however, a few species in which it is 

 less apparent, the skin having no greater consistence than that 

 of thin leather ; but these are associated with the more typical 

 forms, on account of their similarity in internal structure. 



Although the Echinodermata would appear at the first glance 

 to be the most typical group of the Radiate animals, if we con- 

 sider the radiate structure as the primary characteristic of this 

 division, recent researches have tended to render this view rather 

 doubtful, and to indicate, either that the Echinoderms form a 

 very aberrant section of this fourth division of animals, ap- 

 proaching the Articulata, or that they are to be transferred 

 bodily into a position more in accordance with their close alliance 

 to the Worms. This view is supported by Professors Leuckart 

 and Huxley, by whom the following Classes are regarded as con- 

 stituting an independent division of the animal kingdom, charac- 

 terized by the gelatinous nature of its members, by their possess- 

 ing usually a digestive cavity with a single opening, and by 

 the constant presence of peculiar organs denominated thread-cells 

 ( 1127) in their skin. For the group thus formed Leuckart 



