ECHINODERMATA. 



147 



arrangements of their muscular fibres, and has described the 

 action by which they either attach themselves to the sur- 

 faces of rocks, or effect their sluggish movements.* 



§ G. Ecldnodermata. 



Ascending in the scale of organization \vc come to the 

 Echinoderinata, a class which comprehends the families of 

 the Jislerida^ the Echinida, the Holothitrida, and the Cri- 

 noidea, together with other tribes of less note. 



These animals, both in their general form, and in the ar- 

 rangement of their internal organs, retain, in a very marked 

 manner, the radiated disposition so characteristic of Zoo- 

 phytes: for we find all their parts symmetrically arranged 

 either in lines, or in compartments, which proceed from a 

 common centre, or axis, and which are repeated, in regular 

 succession, all round the circumference (See Fig. SS to 94.) 

 Besides an external horny, or semi-calcareous covering, 

 there is also provided, for the support of the softer parts, a 

 kind of internal skeleton, or jointed frame-work. The or- 

 gans in the interior of the body are farther supported by 



* M^molres de TAcacIemie des Sciences, 1710, p. 490. 



