12G 



BIOLOGY: GENERAL AND MEDICAL 



nu. 



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 

 [ c motion, locomotion, and pre- 

 hension. 



The unicellular organ- 

 isms show the most primi- 

 tive of these in the pseudo- 

 pods of the amreba, 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 



3ESJfcSSi. fc JS3 1!a ' <- ertain of the bdn 



organ is developed; en, the cnidocil or adapted to locomotion, and 



"trigger"; cp , the capsule or nettling certain arranged in such 



organ; /, the nettling filament or lasso; ] 



. neck of the capsule; nu. nucleus Banner as to direct CU1'- 



oftheceiL rents of fluid toward the 



oral orifice of the organism. 



FIG. 43. Nettling cells of Hydra. 

 (After Schmeil.) 



