X.] THE FRESH- WATER POLYPES. 101 



3. Examine with a high power: try to make out the 



different cells of the ectoderm — 



a. Large somewhat conical nucleated cells, with 

 the broader end turned outwards. 



j8. Smaller rounded cells packed between the deep 

 ends of the larger ones. 



.7. The nematocysts : small oval capsules, with a 

 filament coiled up inside them, which are 

 dispersed through the ectoderm in the interior 

 of its component cells. 



4. Treat with magenta: note the staining of the cells, 

 the emission of the thread-cells, and the protrusion 

 of their threads : three chief forms of thread-cell — 



a. An oval capsule with a filament many times its 

 own length attached to one end, and three 

 short processes radiating from the base of the 

 thread. 



^. Smaller thread-cells, without the radiating pro- 

 cesses and with a short thread. 



7. Cells like /3. but with a much longer thread. 



5. Imbed in paraffin a Hydra which has been hardened 

 in chromic or osmic acid* and cut sections from it; 

 or lay a prepared Hydra on a glass slide and with a 

 razor cut off transverse slices; having obtained by 

 either method a number of thin sections mount them 

 in glycerine and make out — 



1 When a Hydra is placed in the above hardening fluids it nearly 

 dways contracts so much as to make it difficult to cut sections. If it be 

 first killed, by placing it in a small quantity of water and when it has ex- 

 panded adding some boiling water, fairly extended specimens for hardening 

 Ban usually be obtained. 



