THE HYDRA. IJ 



Exercise i. Make at least two drawings of the Hydra on an enlarged 

 scale showing the animal in an extended and in a contracted position 

 respectively. 



Technical Note. Mount one of the polyps on a slide, supporting the 

 cover glass in two places with small clippings of thick paper. Study 

 with a higher power of the microscope. 



By focusing sharply on the margin of the body or the tentacles you 

 will be able to make out two layers in the wall, an outer ectoderm 

 and an inner endoderm. Between these is a thin supporting layer, 

 the mesogloea, without cellular structure. In which layer is the 

 coloring matter? 



Examine now the knots on the tentacles cnidoblasts more care- 

 fully. Each contains a highly refractive stinging organ or nematocyst, 

 the essential parts of which are a coiled hollow thread and a short 

 spine-like trigger, the cnidocil. You may see some of the latter 

 projecting beyond the general surface. If a small crustacean, as 

 Daphnia or Cyclops, blunders against one of these triggers the coiled 

 thread shoots out and paralyzes the intruder by the injection of a small 

 quantity of poisonous fluid. The prey is then conveyed to the mouth 

 by means of the tentacles. 



Distended regions of the body or the tentacles probably contain 

 particles of food which the endoderm cells lining the gastro-vascular 

 space have not yet had time to deal with. Some of these cells secrete a 

 digestive fluid, while others by means of flagella or pseudopods ma- 

 nipulate the food and cause it to circulate from one cell to another. 

 Bear in mind that the hollow space in the tentacles is merely an out- 

 pocketing of the central body cavity. The same is true of the buds, 

 which are thus supplied with food until separated from the parent 

 polyp. 



By removing the supports of the cover glass the weight of the latter 

 will so compress the Hydra that certain details may be more clearly 

 noted. Run a little weak acetic acid under the cover glass and then 

 look for discharged threads of the nematocysts. 



