ON POLYPS. 



by microscopical researches to possess the cellular structure apper- 

 taining to vegetable organization, and are thus placed beyond the 

 limits of our present investigations. 



(26.) We have hitherto spoken of animals which do not appa- 

 rently possess any stomach or oral aperture, any apparatus for 

 the purpose of the digestion or prehension of food. Before describ- 

 ing the more complex forms of polyps, we will now select a group 

 of that class of animals, in which the organs provided for these pur- 

 poses are easily recognisable ; and, as the simplicity of their orga- 

 nization will well exhibit the principal points in the physiology of the 

 acrita,we shall detail at some length the facts known concerning them. 



The HYDR.E, or fresh- water polyps, are common in the ponds and 

 clear waters of our own country ; they are generally found creeping 

 upon confervse which float upon the surface, and may readily be pro- 

 cured in summer for the purpose of investigating the remarkable cir- 

 cumstances connected with their history. p^ 4 



The body of one of these 

 simple animals consists of a 

 delicate gelatinous tube, con- 

 tracted at one extremity, which 

 is terminated by a minute 

 sucker, and furnished at the op- 

 posite end with a variable num- 

 ber of delicate contractile fila- 

 ments, placed around the open- 

 ing which represents the mouth. 



In the Hydra viridis, (jig- 

 4, 1,) the species most common 

 amongst us, the tentacular fila- 

 ments are short, and, when elon- 

 gated to the utmost, are not 

 equal to the length of the body; 

 but in the long-armed species 

 Hydra fusca, (Jig. 4, 2,) they 

 are much prolonged, and of extreme tenuity. If placed in a small glass 

 tube, one side of which is flattened, these animals may readily be sub- 

 mitted to microscopical examination, and, from their transparency, 

 their entire structure is easily made out. When highly magnified, the 

 whole body is seen to consist of a granular substance, generally of a 

 greenish hue, the granules being loosely connected by a semifluid 

 albuminous matter ; but the most minute research reveals no fur- 



