1 8 AMCEBA. 



myriads of these microscopic organisms must be present in the 

 challc rocks of our North and South Downs alone, when we 

 consider that thousands go to the square inch ! 



The Protozoa that have a definite cell -wall are sometimes 

 called Infusoria (fig 3, i., ii., iii.) : these and the shell-bearing 

 Foraviinifera (fig. 3, iv. and v.) are of definite form. The In- 

 fusoria are quite unable to alter their shape, but some of the 

 Foraminifera have the power of throwing out long thread-like 

 processes or pseudopodia through minute perforations in the 

 shelL One of the simplest Protozoa is known as the Proteus- 

 Animalcule or Amceha (fig. 2). 



AiucBba is a simple, unprotected mass of protoplasm or Bar- 

 code, which may be found in damp earth and in water. In 

 appearance it resembles a small speck of white, transparent, 

 structureless jelly. If this speck is observed under strong 

 magnifying power, it will be seen to move by throwing out 

 little finger-shaped processes, the pseudopodia (Psu). This 

 simple organism is apparently composed of two layers, a 

 granular layer inside and a clear transparent layer on the 

 outside : the former is known as the endosarc and the latter 

 as the ectosarc. These two layers must not be mistaken for 

 two distinct membranes, for they are continuous, only certain 

 granules collect towards the interior. When the pseudo- 

 podia (Psu) are thrown out we shall see, if we watch care- 

 fully, that the granules flow up the centre of the process as it 

 elongates. Three otlier bodies are to be noticed in this minute 

 creature : first, a small dark oval spot, with a clear border and 

 permanent in shape, situated in the endosarc ; this is the nucleus 

 (n), and it will be found to stain dark-red with picro-carmine. 

 There will also be seen contracting and expanding a clear space • 

 this is the so-called " pulsating vacuole " (cv), of which there are 

 two in some forms of Protozoa (Parama'cium). The pulsatin" 

 vacuole is said to be an excretory organ ; uric acid has been 

 extracted from these minute cavities. Lastly, there are present 

 a number of so-called "food vacuoles" (Pv), spaces surround- 



