100 ELEMENTARY BIOLOGY. [x. 



. The testes : small conical colourless eminences 

 below the point of attachment of the tentacles. 



e. The ovary: a larger rounded colourless promi- 

 nence near the base : there may be more than 

 one. 



f. The buds: young HydraB, of various sizes and 

 stages of development, attached to the sides 

 of the parent. 



Either 8. e. or f. 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. 



y8. 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 constantly either extending or shortening its 

 body and tentacles, and so altering its form and 

 place. 



/3. Its irritability ; slight pressure or other stimu- 

 lus immediately causes it to contract. 



