ACALEPH.E. 181 



many coloured tentacula, while they are moving in 

 search of food : to observe the quickness with 

 which they seize on whatever prey comes within 

 their reach, and to notice the suddenness with 

 which they collapse into a round contracted mass, 

 on receiving the slightest injury. Yet these animals 

 are not of necessity confined to the particular spots 

 where we see them fixed ; for they are capable, 

 when disturbed, of seeking, by a slow progressive 

 motion, a more secure abode. Reaumur has mi- 

 nutely examined the arrangements of their mus- 

 cular fibres, and has described the actions by which 

 they either attach themselves to the surfaces of 

 rocks, or effect their sluggish movements. * 



§ 6. Echinodermata. 



Ascending in the scale of organization we come to 

 the Echinodermata, a class which comprehends the 

 families of the Asterida, the Echinida, the Holo- 

 thurida, and the Crinoidea, together with other 

 tribes of less note. 



These animals, both in their general form, and 

 in the arrangement of their internal organs, retain, 

 in a very marked manner, the radiated disposition 

 so characteristic of Zoophytes : 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. 

 88 to 94). Besides an external horny, or semi-cal- 



* Memoires de I'Academie des Sciences, 1740, p. 490. 



