600 



THE WONDERS OF GEOLOGY. 



Lect. VI. 



manner in which the tentacula are formed by an extension 

 of the upper margin. The Hydra is endowed with vitality 

 in a very extraordinary degree, and its substance is highly 

 sensitive and contractile in all its parts. It fixes itself to 

 other bodies by the small end of the tube, and expands 

 and contracts at pleasure. These enlarged drawings 

 (Lign. 132) represent polypes in different positions and 



Lign. 132.— Hydiue or Freshwater Polypes. 

 (Drawn by Miss Ellen Maria Mantell.) 



Fig. 1. Hydra fusca magnified, the tentacula partially expanded. 2. Two Hydra; 

 on the same base, one contracted, the other expanded. 3. Hydra viridis, 

 natural size. 4. Hydra, with the body enlarged from its containing food. 

 5. Vertical section of Hydra viridis, highly magnified. 6, 7, 8. Hydra? in 

 various states of progression. V. A double Hydra, produced by the vertical 

 section of a single one. 



states of contraction. The mode of progression is shown 

 in figs. 6, 7, 8 ; it is effected by the bending of the body 

 into a curve, and holding by the tentacula ; the base is 

 then brought into contact with the fixed point, and the ten- 

 tacula are again projected forwards. The Hydra can 



