PROTOZOA. 7 



addition of new layers to its outside (accretion), and such increase 

 may go on indefinitely so long as fresh solution is available. 



An organism, again, has a life-history, passing through a cycle 

 of changes, which either terminate in death or else in loss of 

 individuality (cf. Section on Amoeba). This is not the case with 

 a mineral. 



4. Biogenesis and Abiogenesis. All existing organisms, so far 

 as we know, have been derived from pre-existing individuals by 

 processes of reproduction (biogenesis). It was, however, formerly 

 held that some organisms could spring directly from non-living 

 matter (abiogenesis, spontaneous or equivocal generation). This 

 belief was gradually limited to the lowest forms of life, and even 

 for them has now been disproved. A flask partly filled with 

 broth or hay-infusion soon swarms with such organisms if left 

 freely exposed to the air ; but if all germs are killed by continued 

 boiling, and the entry of fresh ones prevented by plugging the 

 neck of the flask with sterilized cotton-wool, none of them make 

 their appearance. Such experiments, however, do not prove that 

 abiogenesis has not occurred during some former period of the 

 earth's history. 



CHAPTER I. PROTOZOA. 

 1. AMCEBA (Proteus Animalcule). 



THIS is a microscopic animal, varying much in size. It is found 

 on the surface of the mud in fresh-water pools, on damp earth, in 

 organic infusions, and elsewhere. 



MORPHOLOGY. 



1. External Characters. The form of an active Amoeba is 

 constantly changing, but it is always irregular. The semi-fluid, 

 transparent protoplasm of which the body is wholly composed is 

 thrust out into bluntish lobes (pseudopodia), which vary contin- 

 ually in number and shape. In most Amoebae these can be 

 emitted from any part of the body. 



