86 PROTOZOA. 



framework of lime, flint, or other ??mteriai, while within the 

 cell there is a special kernel or nucleus, or there may be several. 

 There are also other less constant structures. A Protozoan 

 multiplies by dividing into two daughter units, or into a large 

 number ; and tivo individuals often unite temporarily or per- 

 manetttly, in conjugation, ivhich 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, ioivards the Metazoa. 



TYPES OF PROTOZOA. 



First Type — Amceba. 



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

 the outflowing processes of living matter are blunt and finger- 

 like (Lobosa). 



Description. — Ama'ba p?vteus and some other species are 

 found on the muddy bottoms 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 like a little sac of jelly, and glides over 

 the surface of stone and plant by protruding and retracting 

 blunt processes or pseudopodia. As they move the shape 

 constantly changes, whence the old (1755) popular name of 

 " Proteus animalcule." Round the margin, which sometimes 

 shows an apparent radial striation, the cell substance is firmer 

 and clearer than it is in the interior, which is granular and 

 more fluid. According to Professor Ray Lankester, the 

 formation of pseudopodia is due to the outflowing of the 

 central fluid substance at places where the firm outer pellicle 

 has been temporarily ruptured. In the centre of the cell lies 

 a single nucleus, and Ama'ba princeps has numerous nuclei. 

 The food consists of minute Algse, such as diatoms, or of 

 vegetable debris. It is surrounded by the finger-like pro- 

 cesses, and engulfed along with drops of water,Vhich form 

 food vacuoles in the cell substance. After the digestible 

 parts of the food have been absorbed, the undigested residue 

 is got rid of at any point of the protoplasm. "Oae or more 

 contractile vacuoles are visible in the cell substance. They 



