PROTOZOA. 



PROTOZOA. 



The Protozoa have already been alluded to as animals consisting of 

 a single cell. Their form is so minute that they can only be seen with 

 the microscope, and hence they have received no common name, if we 

 except the term animalcule, which is applied indiscriminately to any 

 minute animal or plant. All Protozoa need an abundance of moisture 

 for their existence ; and while a few live as parasites in other animals, 

 and still fewer in moist earth, the great majority are aquatic, abounding 

 in the sea as well as in fresh water. There is a common idea that under 

 a microscope every drop of water is seen to be teeming with animalcules, 

 — an idea which doubtless had its origin in some of the pictures in which 

 the artist had combined as many forms as possible. Fortunately, animal- 

 cule are not so common as these illustrations would indicate, and yet no 

 one need have any difficulty in rinding them, for they abound in the ooze 

 lying at the bottom of stagnant water, or in the muddy shores of the 

 sea. Good drinking-water is comparatively free from them. 



One of the common forms, certainly the one best known, at least by 

 reputation, is the Amoeba, a minute jelly-like form found both in salt 

 water and in fresh, and protean in shape. It forms a favorite and 

 valuable object for microscopic study, for in its 

 atomy one has a whole epitome of physiology. 

 When first put upon the microscope slide the 

 disturbance makes it contract into an approxi- 

 mately circular form, but soon it begins to move ; 

 a slight prominence forms on one side, and 

 towards this the jelly, or, better, protoplasm, 

 of which it is formed, begins to flow, the promi- 

 nence soon becomes a lobe, and finally it becomes 

 the animal itself, the former position of the body 

 being indicated by a small prominence like that 

 with which we started. Thus it travels. On its 

 course it meets with something digestible, and 

 swallows it. It has no hands to capture its prey, 

 no mouth to put it in ; it simply crawls, or seems 



Fig. 2. — An Amoeba eating a Spi- 

 rogyra (s). n, nucleus; c. con- 

 tractile vacuole; d, a diatom 

 which has been ingested. The 

 arrow shows the direction in 

 which the Amoeba is crawling. 



