HYDRA. 53 



radial symmetry (the arrangement of " like 

 parts about an axis from which they ra- 

 diate "). 



6. Buds. (Often present.) 

 Make simple outline sketches of the hydra when 



extended and when contracted. 

 II. Structure. 



Transfer ~by means of a pipette to a slide and 

 mount in water, supporting the cover-glass at 

 each side ~by another cover-glass. Study with 

 AA 2. 



Review the parts examined with a hand-lens. 

 Observe also : 



1. The transparent colorless outer layer, the 



ectoderm. 



2. The inner thicker layer, the entoderm* (green 



in H. viridis). 



With D 2 examine the character of the ectoderm 

 (best seen in extended tentacles). Find : 



3. The knob-like prominences, the nettle-bat- 



teries, f consisting of numerous cnidoblasts 

 (parent-cells of nematocysts), each with a stiff 

 projecting process, cnidocil. 

 Each cnidoblast contains a highly refractive 

 body enclosing a thread (nematocyst). 

 Kinds of nematocysts : 



* The green hydra shows best the distinction between ectodermal 

 and entodermal cells, especially when macerated. 

 \ Best seen in the brown hydra. 



