OF MICRO-ORGANISMS. 55 



cle, which in a state of rest stays extended. Infusoria 

 placed in a preparation where they are at their ease, 

 swim quietly about; if any sharp excitation disturb 

 them, they accelerate their pace; those armed with a 

 rigid bristle at the posterior extremity, rush precipi- 

 tately onward whenever another Infusory chances to 

 touch that tactile appendage. The unaggressive Par- 

 mecia, when attacked, endeavor to escape, but are 

 also able to defend themselves by means of the tricho- 

 cysts with which their ectosarc is armed. 



IV. 



Unicellular organisms do not all live in a detached 

 state; a large number of species are found grouped 

 together in colonies; the initial basis of these agglom- 

 erations is always a mother cell, the offspring of which 

 instead of dispersing to live at large, remain aggluti- 

 nated to one another. Ehrenberg had believed that 

 in certain species (especially in the case of the Antho- 

 physa vegetans, an aggregation of minute monads 

 growing as a sort of bush) the colony was created by 

 the union of minute organisms that originally lived at 

 large; but observation has shown that his theory was 

 incorrect. It may be laid down as a general rule that 

 every colony of monocellular animals or vegetables 

 spring from the divisions of a single cellule. The 

 cellules of one and the same colony, therefore, are 

 always sister cellules, and the colony represents a 

 family in miniature. 



A leading instance of a colony wholly temporary, 

 is found in those organisms the cuticle of which does 

 not take part in the phenomena attending the division 

 of the protoplasm. In this case, the protoplasm beneath 

 the envelope alone divides; the segments resulting 



