HYDRA. 55 



light. Search for the following kinds of cells, and 

 sketch those clearly observed : 



1. Entoderm-cells, of very transparent proto- 



plasm, containing in H. viridis numbers of 

 Zoochlorella cells. 



2. Interstitial cells : small angular granular 



cells with distinct nuclei : 



3. Cnidoblasts containing nematoeysts ; look 



for the cnidocil and the nucleus. 



4. Large ectoderm-cells : pale cells with large 



nuclei ; the bases drawn out into muscular 

 processes. 



5. Nerve-cells : small granular cells with long 



branches and distinct nuclei. 



6. Gland - cells (entodermal) : columnar cells 



with distinct nuclei and foam-like, granu- 

 lar protoplasm. 

 B. Prepared Sections.* 



With AA 2 observe in transverse section : 



1. General form. 



2. Ectoderm. 



3. Entoderm. 



4. Supporting lamella. 



* For sections the hydras may be immobilized in a .25 per cent, 

 aqueous solution of hydroxylamin in from fifteen to sixty min- 

 utes, and then preserved in picro-acetic acid ; or they may be 

 killed instantly in the extended state in a few drops of water in 

 a watch-glass by pouring upon them hot, saturated aqueous solu- 

 tion of corrosive sublimate. Fine preparations may also be made 

 fiom hvdras preserved in osmic acid. 



