PHYLUM CCELRNTERA. 



varying action of the environment. Its cells are pigmented, often 

 vacuolated, and most of them are either flagellate or amreboid. The 

 pigment bodies in H. viridis seem comparable to the chlorophyll cor- 

 puscles of plants ; in H, jusca they are brownish and without chloro- 

 phyll. The active lashing of the flagella causes currents which waft 

 food in and waste out. If some small animal, stung by the tentacles, 

 is thus wafted in, it may be directly engulfed by the amoeboid processes 

 of some of the cells, and it has been noticed that the same cell may be 

 at one time flagellate and at another time amoeboid (cf. the cell-cycle, 

 p. 104). After this direct absorption the food is digested within the 

 cells, and while some of the dark granules seen in these cells may be ' 

 decomposed pigment bodies, others seem to be particles of indigestible- 

 debris. Thus Hydra illustrates what is called intracellular digestion, 

 such as occurs in Sponges, some other Ccelentera, and some simple 

 "worms." But experiments show that some of the food may be 

 digested in the gut cavity, and subsequently absorbed. Thus it seems 

 that both intracellular and extracellular digestion occur. 



Some of the endoderm cells have muscular roots like those of the 

 ectoderm. They lie on the inner side of the middle lamina, in a trans- 

 verse or circular direction. A few cells near the mouth and base are 

 described as glandular, and the presence of a few stinging cells has 

 been recorded, though some suggest that the last are discharged ecto- 

 dermic nematocysts which have been swallowed. 



The middle lamina, representing the mesoglcea, is a thin homogeneous 

 plate, bearing on its outer and inner surfaces the muscular roots of 

 ectodermic and endodermic cells (Fig. 65, D). 



It is historically interesting to notice the important step which was 

 made when, in 1849, Huxley definitely compared the outer and inner 

 layers of the Ccelentera with the epiblast and hypoblast which embryo- 

 logists were beginning to demonstrate in the development of higher 

 animals. Not long afterwards, .Allman applied to the two layers of 

 hydroicis the terms ectoderm and endoderm. 



The division of labour among the cells of Hydra is not very strict, 

 but already the essential characteristics of ectoderm and endoderm are 

 evident. We may summarise these as follows, comparing them with 

 the characteristics of epiblast and hypoblast in higher animals : 



OUTER LAYER. 



MEDIAN LAYER. 



In Hydra the ectoderm None \\\ ^Hydra, apart 

 forms , from the middle lamella. 



Covering cells, stinging 

 cells, nerve cells, muscle 

 cells, etc. 



The embryonic epiblast 

 of higher animals grows 

 into epidermis, nervous 

 system and essential parts 

 of sense organs. 



The mesoblast of higher 

 animals becomes muscu- 

 lar, connective, and skele- 

 tal tissue. 



INNER LAYER. 



In Hydra the endoderm 

 forms 



Digestive cells lining 

 the food canal, and also 

 muscle cells, etc. 



The embryonic hypo- 

 blast of higher animals 

 always lines the digestive 

 part of the food canal. 



