120 



LECTURE VII. 



powers of some of the species, in modifying 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 modifications of the alimentary- 

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

 Hydrozoa*, digestion is performed by 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 

 j 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 Hydra], 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 cells are condensed, and elongated in the 



Hydra Aisca. 



• Nat. size. 



* LXXXrV. p. 82. Dimorphaea of Ehrenberg ; Sertulariens of Milne Edwards ; 

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

 E. Forbes. 



t " In Africa et Arabia Hydras studiose frustra quajsivi," CXVIL, p. 292. 



