VI.] THE FRESH-WATER POLYPES. 349 



c. The sex-organs^ if present; colourless prominences, 

 variable in number and position (cf. Sect. 8). 



S, e may be one or more absent, or all may coexist, 

 in individual specimens. 



b. Structure. 



a. The animal evidently composed of two layers, an 

 outer transparent, ectoderm^ and inner, endoderm; the 

 latter alone containing chlorophyll in the green 

 species, or such colouring matter as is present in 

 the brown one. The et:toderm 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. 



yS. The body-cavity ; least obvious in the green species, 

 frequently visible in the brown ones as a darker 

 central area with which the mouth-opening is con- 

 tinuous. Note that it extends into the tentacles; 

 corpuscles can, with care, be seen floating within 

 these, when 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. 



y8. Its irritability ; slight pressure or other stimulus im- 

 mediately causes it to contract. 



3. The tentacles and nematocysts. Attention has al- 

 ready been called to the knob-like eminences of the former. 

 Note that these are widely separated in extension ; closely 

 applied in contraction. Examine one of them under a high 

 power, with the tentacle fully extended. 



