120 



LECTrEE VU. 



powers of some of the species, in modif jing the crust of the earth, is 

 neither slight nor of limited extent. 



This great division of the radiated animals is divided into three 

 groups or classes, according to the modificatious of the alimentary 

 canal. In the first and lowest organised class, which I have called 

 Hydrozoa*, digestion is performed hy the secretion of a simple sac, 

 excavated in the gelatinous and granular parenchyme of the body. 

 In the second class, called Anthozoa, the digestive sac, which, like 

 the first, throws out the rejectamenta by the same aperture as 



that which receives the 

 nutriment, is suspended 

 by a series of vertical 

 folds of membrane, in 

 a distinct abdominal ca- 

 vity, to the outer parietes 

 of the body. In the 

 ) third and highest class, 

 called Bryozoa, the ali- 

 mentary canal, which is 

 suspended loosely in an 

 abdominal cavity, is pro- 

 vided, as has been already 

 stated, with a distinct 

 mouth and anus. 



It is remarkable that 

 the most locomotive of the 

 Polype tribe is at the 

 same time the type of the 

 lowest organised group. 

 The nydra\, or common 

 fresh-water Polype {fig. 

 60.) consists, when mag- 

 nified even with a mode- 

 rately high power, appa- 

 rently of a granular sub- 

 stance of a greenish or 

 reddish hue, the granules or cells being loosely connected by a semi- 

 fluid matter. The external ceUs are condensed, and elongated in the 



Hxdra fiisca. 



• Kat. size. 



* LXXXIY. p. 82. Dimorphaa of Ehrenberg ; Sertuhriens of MUne Edwards ; 

 Ifudibrachiata of Farre ; Hydroida of Johnston ; Hydraida and Sertulariada of 

 E. Forbes. 



t " In Africa et Arabia Hydras sradiose frustr a qnaesivi," CXTTL, p. 292. 



