to2 ELEMENTARY BIOLOGY. [CHAP. 



LABORATORY WORK. 



1. Put into a beaker some water containing bodies to 

 which Hydras are attached, and place the beaker in a 

 window not exposed to direct sunlight : in the course 

 of some hours many Hydras will be found attached 

 to that side of the glass which is turned towards the 

 light. Note their size, form, colour, mode of attach- 

 ment and movements. 



2. Transfer a Hydra, by means of a pipette, on to a 

 slide; cover in plenty of water with a large coverslip, 

 and examine with i inch obj. Note 



a. Form. 



a^ The base (so called foot) : a flattened disc : nar- 

 rower or wider than the body according to the 

 state of extension of the latter. 



(3. The body proper : cylindrical, varying much in 

 length and diameter with the state of extension 

 of the animal; its conical free end, with an open- 

 ing (mouth) in it. It is often difficult to see the 

 mouth in this way, especially in the green species. 

 It is readily seen however if a Hydra be placed 

 in a drop of water, without a coverslip, and be 

 watched with an inch objective until it turns its 

 anterior end up towards the observer. 



y. The tentacles; ranged round the mouth; their 

 number and shape; their varying length and 

 diameter ; the knob-like eminences on them. 



B. The testes; small conical colourless eminences be- 

 low the point of attachment of the tentacles. 



