HYDRA, 



113 



Fig. 47. — Portion of Body-wall of Hydra. 

 (Highly Magnified.) {Ad 7tat.) 



Interstitial Cells. 



Cnidoblast. 



Epitheli^ Cell. — "^S--^^^^^^^?^^ \ Vacuole. 



< Nucleus. 

 Chromatophores. 



Mesogloea. 



The chromatophores are numerous spherical bodies with 

 definite walls. They are bright green owing to the presence 

 of a green pigment called chlorophyll. This chlorophyll is 

 characteristic of the plant kingdom (see Chapter II.), and 

 some have regarded the chromatophores as symbiotic algae 

 living in the tissues of Hydra, The green tint of Hydra, 

 already noticed, is due to these bodies which are seen 

 through the transparent ectoderm. In about the centre of 

 the body the endoderm cells have one or more large 

 vacuoles, containing a clear fluid. The fluid is said to be 

 discharged into the coelenteron and to be digestive in 

 function. Other bodies in the endoderm cells may be 

 recognised as particles of food. The inner ends of the 

 endoderm cells appear to have no cell-wall, and are either 

 produced into several flagella or into amoeboid-like pseudo- 

 podia. 



The ectoderm cells are of two kinds, the larger epithelial 

 cells and smaller interstitial cells. 



The epithelial cells are arranged in a single layer ; each 

 has a definite cell-wall and a nucleus. In most of them the 

 inner end is produced into one or more processes, which 

 are not amoeboid but show fine striation and appear to be 

 specially contractile. They are therefore known as muscular 

 M. 9 



