HYDRA. 113 
Fig. 47.—PorrIoNn OF Boby-waLL oF L[ypRA. 
(Highly Magnified.) (dd maz.) | 
Muscular 
Process. 
Interstitial Cells. 
Cnidoblast. 
Epithelial Cell. 
__Vacuole. 
Nucleus. 
Chromatophores. 
Mesogleea. 
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 alge 
living in the tissues of Aydva. 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 ccelenteron 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 amceboid-like pseudo- 
odia. 
E The ectoderm cells are of two kinds, the larger efithelial 
cells and smaller xéerstitial 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 amceboid but show fine striation and appear to be 
specially contractile. They are therefore known as muscular 
M. 9 
