x.] THE FRESH-WATER POLYPES. 103 



e. The ovary: a larger rounded colourless pro- 

 minence near the base : there may be more than 

 one. 



. The buds: young Hydrae, of various sizes and 

 stages of development, attached to the sides of 

 the parent. 



Either 8, e, or , or all of them, may be absent in 

 some specimens. 



b. Structure. 



a. The animal evidently composed of two layers, 

 an outer, ectoderm, and inner, endoderm, the latter 

 alone containing chlorophyll in the green species : 

 the ectoderm is marked out into areas, and may 

 with care be seen to be composed of distinct 

 cells, though this is a little difficult to make out 

 in fresh specimens. 



P. The body -cavity: difficult to make out in the 

 green species, frequently visible in the brown 

 ones as a darker central patch with which the 

 mouth-opening is continuous; the extension of 

 the body-cavity into the tentacles. Note cor- 

 puscles floating along inside them when they are 

 extended. 



c. Movements. 



a. The general contractility of the animal ; it is con- 

 stantly either extending or shortening its body 

 and tentacles, and so altering its form and place. 



/?. Its irritability ; slight pressure or other stimulus 

 immediately causes it to contract. 



3. Examine with a high power: try to make out the 

 different cells of the ectoderm 



