VI.] THE FRESH-WATER POLYPES. 351 



a. The body-cavity ; in young buds continuous with that 

 of the parent; in buds ripe for dehiscence com- 

 pletely constricted off therefrom. Note that the 

 mouth is formed as a secondary perforation. 



b. The tentacles ; variable in number. A solitary one 

 may appear first, the second and third being sym- 

 metrical with respect to each other. Compare the 

 fully-formed animal. 



The rate of development of the tentacles, indi- 

 vidually or collectively, varies under change of tem- 

 perature, and their order of appearance is somewhat 

 inconstant. There are usually in the fully-formed 

 animal (8 12 hrs.), 6 for H. fusca, 8 for H. m- 

 ridis. 



5. The study of prepared sections. Place one or more 

 Hydras, preferably in full diet, in Kleinenberg's picric 

 acid solution; after two hours' immersion therein, transfer to 

 alcohol of increasing strengths. When fully hardened, stain 

 with borax-carmine, imbed in paraffin and cut into trans- 

 verse sections. Mount in Canada balsam and examine 

 under a low power. 



a. The ectoderm; of uniform thickness and composed, 

 for the most part, of squarish-looking cells which stain 

 lightly. 



b. The endoderm; composing the inner two-thirds or 

 more of the body-wall, extremely variable in thick- 

 ness. Its component cells are highly vacuolated. 



c. The body-cavity (enteroti) ; enclosed by b. Variable 

 in capacity in different sections, in accordance with 

 the state of contraction of the body-wall. 



d. a and b will be seen to be separated by a hard line, 



