126 



BIOLOGY: GENERAL AND MEDICAL 



-nu. 



FIG. 43. Nettling cells of Hydra. 



(After Schmeil.) 



A, Unexploded; B, exploded. 6, Barbs; 

 c, the nettling cell in which the nettling 

 organ is developed; en, the cnidocil or 

 "trigger"; cp, the capsule or nettling 

 organ; /, the nettling filament or lasso; 

 :;, neck of the capsule; nu, nucleus 

 of the cell. 



from antecedent forms of 

 life, and as these are not 

 to be found everywhere, 

 either the animal must 

 wait until such come to it 

 and then seize them, or go 

 in search of them. Thus 

 comes about the necessity 

 which is met by the de- 

 velopment of organs of 

 motion, locomotion, and pre- 

 hension. 



The unicellular organ- 

 isms show the most primi- 

 tive of these in the pseudo- 

 pods of the amoaba, and 

 the cilia and flagella of 

 the infusoria. Pseudopodia 

 subserve all three purposes, 

 motion, locomotion, and 

 prehension, but cilia and 

 flagella are higher special- 

 izations and confine their 

 usefulness to motion, by 

 which stationary cells pro- 

 duce currents in the sur- 

 rounding fluids, and loco- 

 motion by which the cell 

 is propelled through the 

 fluid in which it lives. 



Further specializations 

 also occur in regard to the 

 cilia, certain of them being 

 adapted to locomotion, and 

 certain arranged in such 

 manner as to direct cur- 

 rents of fluid toward the 

 oral orifice of the organism. 



