that glide over the substrate or on vegetation or de- 

 caying organic debris in fresh and salt waters. Most 

 of the soil amebas are members of this group. 

 Amoeba proteiis is one of the largest ( ' ,-,,i of an inch) 

 of the common pond amebas. It has a disk-shaped 

 nucleus, longitudinally ridged pseudopods, and the 

 habit of advancing through a flow of all the proto- 

 plasm into the leading pseudopod. Pseudopods are 

 used not only in moving about but also in engulfing 

 food and in taking it into the body, surrounded by a 

 minute quantity of water that then forms the food 

 vacuole we see within the protoplasm. Anyone 

 tempted to speak of "the simple ameba" should 

 watch one moving about and capturing prey. If the 

 food is a motionless algal cell, the ameba's body 

 flows closely about the alga as a flowing drop of oil 

 might surround a glass bead. But if the food is a 

 rapidly swimming protozoan, something quite dif- 

 ferent occurs. The ameba sends out long pseudopods, 

 in a wide embrace, but at no point in contact with 

 the prey until it has been completely surrounded on 

 the sides and over the top so that it is trapped against 

 the substratum. Only then is it closely enveloped and 

 finally incorporated into the body. Amebas can also 

 tell food particles from nonnutritive ones and show 

 a preference for one species of prey over another. 

 The giant ameba, Pelomyxa carolinensis. has several 

 hundred small nuclei and may be up to i^, of an inch 

 long when moving actively. Though rarely found in 

 nature, it is readily obtainable from biological sup- 

 ply houses and is very convenient to watch because 

 of its large size. It ingests paramecia, one after an- 

 other, as many as twenty in one food vacuole. (The 

 naming of the fresh-water amebas is still being de- 

 bated, quite unknown to the amebas themselves, so 

 that you may find them called by different names in 

 diff'erent books.) 



About half a dozen species of naked amebas live in 

 man; but only one, the dysentery ameba. Entamoeba 

 histolytica, is unquestionably harmful. Small and 

 very active, it is able to dissolve the intestinal lining 

 and to enter the connective tissue and muscle layers 

 of the large intestine; and when present in numbers 

 it causes abscesses, diarrhea (liquid feces) and dys- 

 entery (bloody feces). Human amebiasis is a world- 

 wide disease — not confined to the tropics as many 

 people believe — and it is spread by the contamina- 

 tion of food or of drinking water with the resistant 

 cysts of an ameba that is itself too delicate to be 

 passed around. We do not have immunizing tech- 

 niques for amebiasis, as we do for typhoid, and when 

 traveling in countries where soil is likely to be ferti- 

 lized with human manure, or water contaminated 

 with human sewage, or food handled by people with 

 unsanitary habits, it is best to avoid foods that can- 

 not be peeled or cooked. Ordinary chlorination of 

 drinking water will not always kill the cysts. Ame- 



biasis is better avoided than cured, but we do have 

 several drugs that are effective in most cases. Enta- 

 moeba coli is a harmless commensal that lives in the 

 human colon, feeding mostly on bacteria but occa- 

 sionally on intestinal protozoans that come its way. 

 The mouth ameba. Entamoeba gingivalis, does not 

 form resistant cysts so can only be spread directly 

 from mouth to mouth in eating or in kissing. Even 

 so, by the time they are forty years old about 75 per 

 cent or more of the human population have managed 

 to obtain some of them. These amebas feed on bac- 

 teria and loose cells, and when pyorrhea is present 

 they cluster about the bases of the teeth, probably 

 aggravating the condition. 



Closely allied to the naked amebas of fresh wa- 

 ters are the shelled amebas which have single-cham- 

 bered coverings. The covering may be vase-shaped 

 or bowl-shaped, and has an opening at the bottom 

 through which the lobose, in some cases filose (long 

 and thin), pseudopods are protruded. Some cover- 

 ings are soft and gelatinous; others harden after they 

 are secreted. They may consist entirely of secreted 

 silicious plates or prisms, or they may be constructed 

 of foreign particles, such as sand grains or diatom 

 shells cemented together by a secretion. These 

 shelled amebas live mostly in somewhat foul fresh- 

 water ponds, in sphagnum bogs or peaty soil, and in 

 animal feces. Most often seen is Arcella vulgaris. 

 which lives in the ooze and vegetation of stagnant 

 water and also in damp soil. It secretes about itself a 

 hard, bowl-shaped, yellow or brown transparent cov- 

 ering. Viewed from above, the covering appears 

 circular, and the animal is seen not to fill the interior 

 completely but to be attached to the walls by thin 

 protoplasmic strands. Two nuclei, several contractile 

 vacuoles, and numerous food vacuoles are visible in 

 the protoplasm. Viewed from the side, the hemi- 

 spherical covering is seen to have a concave funnel- 

 shaped opening at the bottom, through which pseudo- 

 pods extend. When an Arcella divides, one daughter 

 inherits the cover; the other has to secrete a new one. 

 Also likely to turn up in organic ooze in fresh water 

 is Diffliigia, which at first glance may be mistaken 

 for a little mass of sand grains. This ameba gathers 

 sand grains and cements them about itself into a 

 pear-shaped (in some species vase-shaped) cover- 

 ing into which it can withdraw completely whenever 

 necessary. 



THE FORAMS 



The "pore-bearers" or foraminiferans (called 

 "forams" for short) are amebas with shells that typi- 

 cally are many-chambered and perforated all over 

 with small pores through which e.xtend long and fine 

 branching pseudopods. These fuse and fork over and 

 over again, forming a spreading network of living, 

 sticky threads that entangle and digest small organ- 



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