GENERAL CHARACTERS OF RADIATA. 449 



which it exercises a most powerful digestion, are nevertheless of 

 an exceedingly simple structure, consisting almost entirely of a 

 membranous bag, with scarcely any traces of special organs. In 

 the higher forms of Polypes, on the contrary, we find a gradually 

 increasing complication ; the stomach forms an inner sac with 

 distinct walls, and special organs for the production of the ova 

 make their appearance. In the larger and more solid Echino- 

 dermata, this complexity of organisation becomes more distinct ; 

 the tissues, instead of consisting of soft and nearly homogeneous 

 membrane, exhibit all the varieties of nerve and muscle, tendon 

 and ligament, distinct vessels and cartilaginous or even bony 

 skeletons. The stomach, instead of being a simple bag into 

 which everything is introduced that comes within the animal's 

 reach, whether it be digestible or not, is but the commencement 

 of a regular alimentary canal, furnished with a set of teeth at its 

 entrance for the reduction of the food, and with glandular ap- 

 pendages for the secretion of the fluids required in the digestive 

 process. And we even find distinct organs appropriated to the 

 aeration of the blood, which sometimes present a very complex 

 arrangement. Now it is remarkable that, with so great an ad- 

 vance in the organisation of the nutritive apparatus, we should 

 find the locomotive and sensorial powers very little developed 

 even in the highest ; so that in this respect they are far surpass- 

 ed by the simpler Articulata, whose general organisation is much 

 inferior. But it must be remembered that the perfection of the 

 locomotive apparatus is the distinguishing or typical character of 

 the Articulata, and that everything is made subservient to it ; 

 whilst conformity with the Plant-like condition seems to be 

 equally the typical character of the Radiata ; being only de- 

 parted from in those higher forms, which conduct us towards 

 other groups. 



1085. In looking at the apparatus, by which the various 

 bodily movements are effected, that are concerned either in 

 obtaining food, or in changing the place of the entire 'body, we 

 observe a considerable diversity in the Radiated classes. In the 

 lowest, the whole tissue appears equally contractile ; whilst, in 

 the highest, a distinct muscular structure exists, in which this 



