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( )l)S('rvo the irregular aj)pc!inincc of tlic surface of the tentacles 

 and body wall. These small elevations indicate the location of 

 hatterics of stinging cells which are found <»n all parts of the body 

 except the basal disc but are most numerous on the tentacles. 

 The high power of the microscope reveals each battery as a com- 

 pact uroup of specialized cells called niidublasts within each of 

 which is the stinging nux-hanism or nenidtocyst in the form of a 

 pear-shaped capsule. Prepared slides should be provided for 

 this observation. 



Determine the extent of the digestive or gadrovaandar cavity. 

 By focusing carefully on such a partially transparent organism 

 .some of the details of internal structure can be determined by 

 bringing into focus different levels of the body. By thus mani- 

 pulating the microscope the two body layers may be recognized. 

 The hollow central cavity of the animal appears Ughter than 

 t he tlenser edges. Ingested food is forced down into the digestive 

 cavity where the greater part is apparently engulfed by pseudo- 

 podia of the cells lining the cavity and digested within these 

 cells in the the protozoan manner. Although digestion is mainly 

 intracellular there is also a limited amount of extracellular diges- 

 tion in Hydra similar to that obtaining in the frog and higher 

 animals. Certain cells among those lining the digestive cavity 

 secrete juices containing enzymes which are discharged upon 

 their surfaces where small amounts of food materials are split 

 into simpler compounds. Undigested food is ejected through 

 the mouth. 



Both agamic and gamic reproduction occur in Hydra. The 

 common agamic mode is that of budding, according to which a 

 new individual is formed as a bud growing from the side walls 

 of the parent. The buds may be found in varying stages of 

 development. When mature the new individuals separate from 

 the parent, leading an independent though sessile life. Trans- 

 verse and longitudinal fission of the whole animal have been 

 reported. 



In its gamic reproduction Hydra is hermaphroditic and without 

 permanent gonads. During the period of gamic reproduction 

 the gonads appear as elevations of the surface. The spennaries 

 form conical elevations in the ectoderm somewhat nearer the 

 tentacles than the base of the animal, while the ovaries form low, 

 broad elevations near the base. Both sperm and egg mother 

 cells ari.se from interstitial cells and are borne by the same 



