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 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. 



y3. 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. 



u. 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. 



/3. 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. 



