104 ELEMENTARY BIOLOGY. [CHAP. 



a. Large somewhat conical nucleated cells, with the 

 broader end turned outwards. 



ft. Smaller rounded cells packed between the deep 

 ends of the larger ones. 



y. The nematocysts: small oval capsules, with a fila- 

 ment coiled up inside them, which are dispersed 

 through the ectoderm in the interior of its com- 

 ponent 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. 



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

 cesses and with a short thread. 



y. Cells like /?, but with a much longer thread. 



5. Imbed in paraffin a Hydra which has been hardened 

 in chromic or osmic acid 1 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 



a. The large and small cells of the ectoderm and its 

 thread-cells, their arrangement and relations. (3). 



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

 always 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 

 expanded adding some boiling water, fairly extended specimens for 

 hardening can usually be obtained. 



