HYDRA; Hydra viridis or H.fusca. 



Examine a living hydra with the naked eye or with a pocket lens, and note the 

 cylindrical body with one end attached by means of a (fisc (foot), and the other 

 sixrrounded by long active tentacles six to ten in number. Observe that the body 

 changes its form, being sometimes elongated with all the tentacles extended, while 

 at other times it is contracted to form a short thick column with the tentacles re- 

 duced to rounded nobs. Observe, further, that when elongated, the tentacles move 

 about apparently in search of food, and that when small Crustacea, &c., are secured 

 they are conveyed to the oral aperture situated in the centre of the oral cone. If 

 there are any buds present, note what stage of growth they have reached. 



A. Place a living specimen in a watch glass in a drop of water and examine with 



a low power, noting again the body, tentacles, oral cone, foot, and buds if pre- 

 sent, and also — 



1. That the mouth leads into a central cavity, which, though closed posteriorly, 



extends into the tentacles, into which food particles freely enter. 



2. That the walls of the body and tentacles consist of two layers, an outer layer 



(ectoderm), and an inner layer (endoderm), which in H. viridis contains 

 chlorophyll granules. 



3. The genital sacs, if present, the testes projecting from the body immediately 



behind the tentacles, and the ovaries from the body near its base. 



B. Make a sketch showing the hydra in an extended and in a contracted state, 



and indicate in the latter sketch all the structures referred to above. 



