(1) Amoeba proteus

An example of Rhizopoda is Amoeba proteus (Rust, fig. 12 a-b; C & V fig. 4.9-4.10) - With a dissecting microscope select one from a culture with a micropipette and place them on a glass slide without a coverslip. Use only 1 drop of medium so that it won't overflow off the slide.

(a) Locomotion

Use low power and observe the movement of Amoeba by means of extensions of the cell. These are pseudopodia. Notice the thin granular internal contents of the cell which may be seen flowing into each new pseudopodium as it forms. The cytoplasm of amoeba may be divided into an outer clear region, the ectoplasm, and inner granular region, the endoplasm. Note that the endoplasm is composed of an inner fluid plasmasol and an outer stiff plasmagel. Make four or five outline diagrams showing the formation of pseudopodia in the same protozoan. Indicate with arrows the direction of flow of the cytoplasm in the pseudopodia. Note the uroid, the ruffled posterior end of the organism. When you examine organisms under 40x magnification, be sure to use a cover slip to protect the objective lens. So long as you don't apply extra pressure, the weight of the cover slip will not hinder the movement of the organism.

(b) Detailed Structure

Using high power carefully examine the ectoplasm and endoplasm. Is there any movement of particles in the ectoplasm? Notice the food vacuoles scattered throughout the endoplasm. In some of the larger food vacuoles you may be able to recognize small plant or animal forms. Examine the part of the organism that you can designate the posterior end and locate the contractile vacuole. It is a clear, round sphere. Watch it form and disappear. Time its contractions. What is its probable function? Locate the nucleus which is a rounded, somewhat grayish, granular-appearing structure. Add these details to one of your earlier outline diagrams. Be especially careful to draw them to scale.

(2) Entamoeba : A Parasitic Amoebae in Action

Entamoeba species are found in a wide variety of hosts, from insects to humans. Entamoeba histolytica (C & V fig. 4.11) is one of the more prevalent of the human parasites, living most of the time like a commensal in the human colon, but sometimes, and for reasons that are unclear, these organisms may cause severe disease, amebic dysentery. Entamoeba invadens is very similar to E. histolytica in appearance and habits, but it is found only in reptiles. Its natural hosts are turtles, but when found in heat-stressed snakes, as may occur in captivity, this organism can cause a pathogenic condition.

Both entamoebas can be grown in culture medium. If available, your instructor will prepare a demonstration of E. invadens. These organisms are much smaller than Amoeba proteus, so you must scan the slide at approximately 100X to find them. Center an Amoeba and examine it under the high dry objective. Notice the difference in the type of pseudopodial extension. These small amoebae are very typical of the limax-type which are commonly found as free-living organisms in soils. They generally have a single eruptive type of pseudopodium. This differentiates them from the Amoeba proteus-type. The nucleus may not be obvious, but you will probably see several bright, refractile rice-starch granules which the organism has ingested by phagocytosis.