88 ORGANISMS OF TISSUES 



ever, they give rise as in the ectoderm, to the various cells of the 

 inner layer. 



C. THE SUPPORTING LAMELLA. The mesogloea, separating 

 ectoderm and endoderm and broken only at the mouth, is the 

 only supporting structure of the organism. It is derived from 

 both layers by secretion from the cells which abut against 

 it. It is homogeneous throughout and the muscle processes 

 are slightly embedded in it. 



PHYSIOLOGY 



In its functional activities Hydra stands midway between 

 the unicellular protozoa and metazoa with well denned organs. 

 In protozoa the protoplasm is differentiated for different func- 

 tions, the ectoplasm for locomotion and food getting, the endo- 

 plasm for digestion and assimilation and for the elaboration of 

 various parts of the cell. Hydra is made up of multitudes 

 of cells, most of which are physiologically unbalanced, that is, 

 some one function predominates over all others. Of these there 

 are eight distinct types, while one additional type the formative 

 cells are physiologically balanced. These nine types of cells 

 do not get beyond the tissue stage in differentiation and internal 

 organs or aggregates of like cells and tissues for performing 

 special vital functions are not developed. Some advance in 

 this direction, however, is seen in the tentacles, the mouth and 

 hypostome and the gonads. 



While the apparatus for performing them is relatively simple 

 the vital activities of Hydra are exactly the same as in other 

 living animals and may be grouped under the headings: (a) 

 nutrition; (b) excretion; (c) respiration; (d) irritability and 

 (e) reproduction. 



A. NUTRITION. The food of Hydra consists of any minute 

 living thing in the surrounding water but it seems to be 

 particularly fond of small Crustacea and embryos of various 

 water-dwelling animals. A passing Cypris touches a tentacle 

 and is stung by the poisoned threads of the nettle cells; this 

 poison, called " hypnotoxin " paralyzes the prey while the 



