Campanularian Hydroid. 13 



batteries of nematocysts or " lasso-cells " at varying positions 

 along the tentacles? In the living specimens (or in a living 

 Hydra, held under a light cover-glass and treated with dilute 

 acetic acid), note that each nematocyst has a rounded body, 

 an elongated filament, and, at the base of the latter, a few re- 

 curved spines. 



Make a drawing of an expanded and of a contracted 

 hydranth. 



Internal Anatomy. If a clean, living specimen is 

 selected there will be little difficulty, by focussing through 

 the transparent perisarc, in making out the internal anatomy. 

 If alcoholic specimens are used, portions of the colony should 

 be stained, cleared, and mounted as described in the Appen- 

 dix. Observe that the perisarc of the hydrocaulus contains 

 a fleshy axis or ccenosarc. The ccenosarc is made up of 

 three layers, and encloses, as a tube, the centrally lying 

 ccelenteric or "body-cavity" Of the three layers the ectoderm 

 is the most external. It is made up of transparent, nucleated 

 cells, and is separated from the innermost and more granular 

 endoderm by a very delicate, third, transparent, supporting 

 layer. 



If the specimen is still alive, note the circulation of the fluid 

 in the body-cavity. How is the circulation brought about? 



Examine one of the tentacles and determine the just de- 

 scribed layers. To which layer are the nematocysts con- 

 fined? The body-cavity does not extend into the tentacles. 

 Demonstrate, if you can, the presence of the three layers in 

 the body-wall of the hydranth. Is the cavity of the hydranth 

 in direct communication with the cavity of the hydrocaulus? 



Make a drawing of the stem, of a tentacle, and of a 

 hydranth, illustrating the three layers. 



