442 CILIARY ACTION. CLASSIFICATION OF RADIATA. 



the base remaining fixed ; so that the whole cilium describes a 

 cone, of which the apex is at its p^int of attachment. 



996. The movement of the cilia appears to be, in some 

 instances, completely under the control of the animal ; and in 

 other cases, to be quite independent of the will, being seen after 

 death, and proceeding with perfect regularity in parts separated 

 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. 



997. The Classes which exhibit the Radiated disposition of 

 parts most distinctly, and which are, therefore, unquestionably 

 to be 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 characteristic 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, regularly 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 recog- 

 nised, and are well known. There are, however, a few species 

 in which it is less apparent, the skin having no greater con- 

 sistence than that of thin leather ; but these are associated with 

 the more typical forms, on account of their similarity in internal 

 structure. 



II. ACALEPH^E (sea-nettles), the Jelly-fish tribe. These 

 animals all live singly, and are well-known inhabitants of our 

 seas. They consist of an extremely soft gelatinous tissue ; and 

 this is rarely supported by any harder substance. They float 

 freely in the water ; and some of them have considerable locomo- 

 tive power. The common Medus(B exhibit the radiated cha- 

 racter with great distinctness. In others it is less apparent. 



