AMGEBA. 87 
movements are effected in any direction on a flat surface. 
Lastly, the treacle processes cannot be again withdrawn 
unless the inclination of the surface be reversed, whereas 
the processes of Ameba may be, and often are, withdrawn. 
These processes are termed pseudopodia. If the flow of 
protoplasm be maintained in one particular pseudopodium, 
it results in a locomotion of the whole animal. 
The body is not the same throughout in appearance and 
structure. A thin superficial layer of more dense and clear 
protoplasm, the ectop/asm, can be distinguished 
from the more fluid interior formed of exdo- 
plasm. In the endoplasm we can discern numerous 
bodies. The majority of these are food-particles which lie 
in small spaces, called /ood-vacuoles, but in addition we 
can usually recognise the uclews and the contractile vacuole. 
The zucleus lies loosely in the endoplasm and appears in 
the living animal like a clear glassy sphere. It consists 
of nucleoplasm differing somewhat in composition from 
protoplasm. 
_ The contractile vacuole is a large spherical space filled 
with colourless fluid and always lies in or immediately 
below the ectoplasm. It gradually expands in size and then 
its walls suddenly contract. A temporary passage or duct 
is formed through the ectoplasm to the exterior by which 
the fluid is extruded. The same process is then repeated. 
This contractile vacuole is usually interpreted as an 
excretory organ for removing waste nitrogenous matters. 
Lastly, there are scattered throughout the endoplasm minute 
granules, the meaning of which is not known, small regular 
crystals, and particles of debris such as sand grains. 
If Ameba be subjected to a rise in temperature the 
movements become more and more active, but when a 
temperature of about 35°C. is reached the pseudopodia are 
withdrawn, the animal assumes a contracted spherical shape, 
and at about 40°C. it perishes. Ameda also reacts to chemical 
and electrical stimuli, but in every case the whole protoplasm 
reacts. In other words, there are no definite sense-organs 
nor nervous system. : 
We have already seen the method of locomotion by 
pseudopodia. When Ameda in the course of its slow 
Structural. 
