78 



or two or three flagella which project from the thicker end 

 into the coelenteron. 



Between the inner endoderm and the outer ectoderm 

 there is a thin supporting lamella, the mesoglcea. 



The ectoderm cells are shorter than the endoderm cells, 

 but they have a similar shape, and their contractile pro- 

 cesses are longer. Since they have muscular processes 

 and are arranged in a layer, they are therefore termed 

 mycepithelial cells. 



Ectoderm Cell. 



Endoderm Cell. 



i acuole 



Nucleus 



Cnidoblast 



Nerve Cell 



FIG. 19 An Ectoderm cell, Endoderm cell, and a Nerve cell 

 connected with a Nematocyst. (After Jickeli.) 



Between the thinner parts of the ectoderm cells, ad- 

 joining the mesoglcea, there are small round interstitial cells. 



Embedded in the ectoderm of the tentacles are the 

 cnidoblasts or stinging cells \\hirh arc seen in clumps stud- 

 ding the tentacles. 



Throughout the ectoderm there are minute nerve cells 

 with ray-like processes, which form a connected nervous 

 network. 



Hydra shortens and lengthens its Body, and it also creeps. 



How are these Movements effected ? 



The ectoderm (mycepithelial) cells have crosspiece 

 processes which contain muscular fibrils lying alongside 



