VI.] THE FRESH-WATER POLYPES. 357 



a. Small organisms; especially Entoinostraca and other 

 small Crustacea. When present one such will gene- 

 rally be found, tightly embraced by the endoderm. 



Examine under a high power and look for evidences 

 of digestion {mfra-enteric) and assimilation. 



b. Smaller organisms; especially Itifusoria^ Diafo7fts, and 

 the like. Frequently to be met with in the central 

 cavity; they may however be found, unchanged or in 

 process of digestion {intra-cellular\ within the indivi- 

 dual cells of the endoderm. 



8. The reproductive organs. (Cf Sect. 2. a. e). 



a. The testes; small conical colourless eminences below 

 the point of attachment of the tentacles, more rarely 

 irregularly scattered over the whole body. 



Gently flatten out a testis in eosin or magenta by 

 pressure on the coverslip, and examine with a high 

 power. According to its state of maturity the follow- 

 ing contents will be found in it — 

 a. A collection of the smaller ectoderm (interstitial) 



cells of variable shape. 

 yS. The same, but having become ovoidal and hyaline. 



y. Cells otherwise like yS, but with a long filament 

 proceeding from them. 



8. Ripe spermatozoa; bodies consisting of a very small 

 oval nucleated head to which a very delicate 

 flagellum is attached, by the movements of which 

 they swim about in the water. They may fre- 

 quently be seen in motion within the unruptured 

 testis. 



b. The ovaries; one or more in number, larger than «, 

 and situated near the base of the polype. 



