82 



LECTURE VII. 



tinal canal, which is bent upon itself, and terminates by a distinct 

 anus opening upon the external surface. The organisation of the 

 polypes is in general very simple, and their faculties or vital phe- 

 nomena seem feeble and inconspicuous. Nevertheless, the influence 

 of their combined powers 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 mem- 

 brane in a distinct abdo- 

 minal cavity, to the outer 

 parietes of the body. In 

 the third and highest 

 class, called Sri/ozoa, the 

 alimentary canal, which is 

 likewise suspended loosely 

 in an abdominal cavity, 

 is provided, as has been 

 already stated, with a dis- 

 tinct 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 

 freshwater Polype {fig' 

 45.), 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 semifluid matter. The external 



* Dhnorphca of Ehrenberg ; Sertulariens of Milne Edwards ; Nudibrachwta 

 of Farre. 



