no PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



have a definite rind, which in a large number (Infusorians) 

 bears motile cilia or flagella, but in the others (Sporozoa) 

 is usually without locomotor structures. But these three 

 phases — amoeboid, ciliate or flagellate, and encysted— may 

 occur in the life-history of one form ; and the three main 

 lines of evolution — Rhizopods, Infusorians, and Sporozoa^are 

 marked by the predominant occurrence of the amoeboid, ciliate 

 or flagellate, or encysted phase of celt life. Many have a 

 skeletal framework— of lime, flint, or other material— while 

 within the cell there is a special kernel or nucleus, or there 

 may be several. There are also other less constant structures. 

 A Protozoon multiplies by dividing into two daughter units, 

 or into a large number ; and two individuals often unite, 

 temporarily or permanently, in conjugation, which is analogous 

 to the union of ovum and spermatozoon in higher animals. 

 A few types, instead of remaining single cells, form by 

 division or budding loose colonies, taking a step, as it were, 

 tozvards the Metazoa, but never forming differentiated tissues. 



First Type of Protozoa — Amceba 



Amoeba, a type of Rhizopods, especially of those in which 

 the outflowing processes of living matter (pseudopodia) are 

 blunt and finger-like (Lobosa). 



Description. — Amoeba proteus and some other species are 

 found in the mud of ponds ; A. terricola occurs in damp 

 earth. Some are just large enough to be seen with the 

 unaided eye. The diameter is often about one-hundredth 

 of an inch. Each is a unified corpuscle of living matter, 

 and glides over the surface of stone and plant by protruding 

 and retracting the pseudopodia. As they move the shape 

 constantly changes, whence the old (1755) name of " Pro- 

 teus animalcule." Round the margin, which may show an 

 apparent radial striation, the cell substance is firmer and 

 clearer than it is in the interior, where it is more fluid, but 

 contains very abundant granules, some of which are of 

 a protein, and others of a fatty nature. In the centre of 

 the cell lies the usually single nucleus. The food consists 

 of minute Algae, such as diatoms, or of vegetable debris. 

 There is reason also to suspect cannibalism. The food 

 is surrounded by the finger-like processes, and engulfed 



