64 



PROTOZOA 



relatively thickest at the anterior end and at the tips of the projecting 

 pseudopodia and is thinnest at the side away from the direction in which 

 the animal may be advancing. Within the endoplasm are seen numerous 

 vacuoles and a nucleus. 



The vacuoles are of three types: (1) Food vacuoles, which appear like 

 colorless drops of water inclosing particles of food. They are spherical in 

 form if the food particle is small and compact but assume somewhat the 

 form of the particle if it is large and elongated in any dimension. Food 

 vacuoles disappear with the egestion of the feces from the body. (2) 

 Water vacuoles, which appear like perfectly transparent colorless drops 



Confracfile 

 vacuole 



Ectoplasm 



Pse udopodium 



Food 

 vacuoles 



Fig. 16. — Amoeba proteus Leidy. X 1000. 



of a spherical shape. They do not change under observation. (3) Con- 

 tractile vacuoles, of which there may be one or more and which appear like 

 the water vacuoles. However, they change in size, gradually growing 

 larger until they reach a maximum, when they suddenly contract and 

 disappear, the process being regularly repeated. Because of this alternate 

 filling and contracting these are sometimes known as pulsating vacuoles. 

 They may sometimes be recognized by the possession of a faint pinkish 

 tint. Throughout the endoplasm may be seen numerous granules of 

 varying shapes and sizes, including crystals, whose nature can be deter- 

 mined by their crystalline form; and foreign substances, like bits of 

 sand. 



Somewhere within the endoplasm, and usually nearer the end away 

 from that which is advancing, is the nucleus. It is finely granular in 

 texture, homogeneous in appearance, and refractive to light, which gives it 

 a brighter or clearer appearance than the surrounding endoplasm. It 

 also frequently has a faint bluish color. 



