CHAPTER XIV 



THE HYDRA AND ITS ALLIES 



HYDRA belongs to the group of Ccelenterata, 1 which 

 includes sponges, and certain organisms with nettling 

 capsules Cnidaria. 2 The Ccelenterata have a more or 

 less radial form, and a system of internal cavities serving 

 for digestion as well as for body cavity. 



The sponges are all sessile, have no nettling capsules, 

 have the body wall perforated by many fine incurrent 

 openings, and a larger exhalant opening (Fig. 192). 



The Cnidaria 3 have nettling organs of some sort. 

 Hydra belongs to this group. 



There are two common species of Hydra ; the one is of 

 a green color (Hydra* viridis 5 ), and the other is flesh- 

 colored (H. fusca 6 ). They are found in standing or slow- 

 running water, attached to submerged plants, sticks, and 

 stones. Throughout the winter they live at the bottom 

 of ponds, below the ice. The body of Hydra is soft and 

 highly contractile, so that, when first drawn from the 

 water, it appears like a speck of jelly. Left undisturbed, 

 the animal expands, and its five to eight tentacles wave 



j, hollow; evrepov, intestine. 



nettle. 



8 A key to the principal subdivisions of the Cnidaria and especially 

 of the Hydromedusse will be found in the Appendix to this Chapter. 

 4 i;5/>a, a mythological monster, capable of regenerating its head. 

 6 Green. Brown. 



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