CHAPTEE II 



PROTOZOA TO WORMS : CELLS, TISSUES, AND ORGANS 



THE first and lowest form in our ancestral series is 

 the amoeba, a little fresh-water animal from T ^- 

 to yo^-d f an i ncn * n diameter. Under the mi- 

 croscope it looks like a little drop of mucilage. This 

 semifluid, mucilaginous substance is the Protoplasm. 

 Its outer portion is clear and transparent, its inner 

 more granular. In the inner portion is a little sphe- 

 roidal body, the nucleus. This is certainly of great 

 importance in the life of the animal ; but just what it 

 does, or what is its relation to the surrounding proto- 

 plasm we do not yet know. There is also a little cav- 

 ity around which the protoplasm has drawn back, and 

 on which it will soon close in again, so that it pulsates 

 like a heart. It is continually taking in water from 

 the body, or the outside, and driving it out again, and 

 thus aids in respiration and excretion. The animal 

 has no organs in the proper sense of the word, and 

 yet it has the rudiments of all the functions which we 

 possess. 



A little projection of the outer, clearer layer of pro- 

 toplasm, a pseudopodium, appears ; into this the whole 

 animal may flow and thus advance a step, or the pro- 

 jection may be withdrawn. And this power of change 

 of form is a lower grade of the contractility of our 

 muscular cells. Prick it with a needle and it con- 



