THE SIMPLEST MANY-CELLED ANIMALS 325 



been removed from an aquarium to some water in a watch-glass. 

 Use a hand-lens. What is the shape of the body ? Does the shape 

 change? Disturb the animal by touching it with a needle. De- 

 scribe what happens. How may this reaction be of use to the 

 hydra ? Notice that one end becomes attached to the glass ; this 

 is the base. Find out if the hydra has a firm hold on the glass dish, 

 e.g., try to wash it off with a gentle jet of water from a pipette. 

 At the free end is a circle of tentacles. How many ? These are used 

 for catching the prey. On the summit of a conical elevation in the 

 center of this circle of tentacles is a small opening, the mouth, which 

 is usually very difficult to see. How do the tentacles behave when 

 the hydra is disturbed ? Can the tentacles move independently of 

 each other? Is this animal bilaterally symmetrical? 



Often young hydras may be seen attached to the bodies of the 

 larger ones, and these are formed by outgrowths or buds (Fig. 97). 



Make an outline drawing of a living hydra, showing as many 

 structures mentioned above as possible (1) fully extended, 

 (2) contracted. 



II. (L) Transfer a hydra, with a little water, by means of a 

 clean pipette, to an object-slide. Be careful to support the cover- 

 glass (small bits of broken cover-glasses may be used as supports) 

 so as not to crush the animal. Examine with low power of the com- 

 pound microscope. Notice that there are two layers of the body- 

 wall ; the outer (ectoderm) is colorless, the inner (endoderm) is green 

 or brown. The green is due to the presence of chlorophyll. In the 

 outer clear layer, look for knob-like swellings, especially on the 

 tentacles. These swellings contain the stinging or nettle-threads, 

 which are organs of defense, and also used for spearing and paralyz- 

 ing water-fleas and other small water animals. 



III. (D or L) Study of Sections. Examine a longitudinal section 

 of hydra with compound microscope and observe : 



The hydra is really a hollow cylinder, the interior of which is 

 the digestive cavity (Fig. 98) . 



The tentacles are hollow outgrowths of the digestive cavity. 



The mouth is the only opening from this cavity to the outside. 



The body-wall is composed of layers of cells on both sides of 

 a deeply-stained line in the preparation. The outer layer is the 

 ectoderm; this is the colorless layer seen in the living hydra. The 

 deeply-stained line (middle layer) is not made up of cells, but con- 

 sists of a gelatinous substance. The inner layer of large cells is the 

 endoderm. It contains the chlorophyll-bodies in green hydras and 

 the brown bodies in brown hydras. 



