24 HYDRA 



ectoderm cells are more columnar in shape, 



contain longitudinal rows of granules, and are 



probably glandular. 



On the outer surface of the ectoderm cells 



there is a thin cuticular layer, more obvious in 



the brown than in the green Hydra. 

 b. The interstitial cells are well seen in sections 



through the body or tentacles, but are absent in 



the foot. 

 0. The nematocysts are very abundant in the tentacles, 



less numerous in the body, and absent in the foot. 



In specimens killed with osmic acid, few, if any 



of them will be discharged. 



2. The mesoderm is seen in transverse sections as a very 



thin transparent lamella, separating the ectoderm 

 and endoderm from each other. 



3. The endoderm cells vary greatly in shape in different 



parts of the body and at different times. During 

 Hfe they are amoeboid, and may extend so far inwards 

 as to almost obliterate the body-cavity of the animal. 

 They contain vacuoles, which are often so large 

 that the cell-substance is reduced to a thin lamella. 

 The nucleus is usually nearer the outer or basal 

 end, and is flattened in shape. The flagella can 

 very seldom be seen in sections. 



In the foot the endoderm cells are much smaller 

 than in the body, and form a row of short columnar 

 or almost cubical cells containing granular concre- 

 tions. 



According to Jiokeli, the endoderm cells of the 

 body-wall have muscular processes which run along 

 the inner surface of the mesoderm lamella in a trans- 

 verse direction, and are connected through its sub- 

 stance with the longitudinal muscle-processes of the 

 large ectoderm cells. 



