CHAPTER XV 

 AMEBA 



A SIMPLE PROTOZOAN 



This animal may serve as a type of the more simple one-celled animals, 

 which are themselves the simplest forms in the animal kingdom. 



92. Occurrence and Appearance. — Amebas occur commonly in fresh 

 water. There are also numerous marine forms. The fresh-water species 

 may be collected in a great variety of situations, such as watering troughs, 

 spring pools, dams, stretches in streams where the water runs over 

 rocky ledges, and wherever there is abundant aquatic vegetation. They 

 are found gliding over the surfaces of submerged plants and of algae- 

 covered mud, rock, or planking. One method which has been suggested 

 for securing them is to collect a mass of pond weed, place it in a flat dish, 

 and cover it with water, A brown scum which gathers on the water in a 

 few days will be found to contain amebas if they were present in the pond 

 from which the weeds were obtained. Not all appropriate habitats con- 

 tain this animal, however, and one frequently has to search for a con- 

 siderable time before coming upon a supply. 



When found, amebas are in the majority of cases only to be discovered 

 by the use of the microscope. The very largest specimens of Amoeba 

 proteus Leidy, however, are visible to the eye as minute whitish dots when 

 seen against a dark background. Under the microscope an ameba appears 

 hke a mass of colorless jelly, irregular in form, and, when active, con- 

 stantly changing in outline. The generic name given to this animal is 

 Amoeba, and under the rules of nomenclature it cannot be changed, but 

 the common name is now quite generally spelled ameba. 



93. Structure. — The animal (Fig. 16) owes the irregularity of its out- 

 line to the fact that from the surface of the main mass extend numerous 

 and variously shaped projections known as pseudopodia. These are 

 constantly varying in length and thickness and may at any time be 

 entirely withdrawn. 



An ameba has no cell wall but possesses an extremely delicate outer 

 layer known as a plasmalemma. This is too thin to be seen even with 

 higher powers of the microscope, but by the movement of particles on its 

 surface it may be shown to exist. Below this is a clear layer of cytoplasm, 

 the ectoplasm, much thicker than the plasmalemma but still forming a 

 thin layer over the surface of the cell. Within this and forming the bulk 

 of the body is the more granular endoplasm. The layer of ectoplasm is 



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