THE SIMPLEST ANIMALS 311 



form, a firm but delicate cuticle over the whole surface, 

 special organs of locomotion (the cilia), special organs for 

 defense (the trichocysts), a definite organ for receiving food 

 (groove and gullet), and differentiation into anterior and 

 posterior ends. Since none of these is found in amceba, 

 paramecium is obviously a much higher type of animal. 

 The amceba is the simplest animal known, and we cannot 

 imagine how it could be made simpler except by the absence 

 of a contractile vacuole. Such an amceba has been described, 

 in which apparently the surrounding water absorbs such 

 excretions as the vacuoles of other amcebas pump out. 



Amceba appears to have no cell-wall or bounding mem- 

 brane. The line visible where the water touches it is simply a 

 line of separation between water and the protoplasm of the 

 animal. If a drop of oil is put into water, there appears to 

 be a line where the oil and water meet ; but there is no special 

 membrane there, simply a surface film between the oil and 

 water. Likewise, we see insects striding on the surface of 

 water, and it. is easy to make a fine sewing needle float on the 

 surface of water ; but no one thinks of the water as covered 

 with a specially differentiated membrane. 



In short, an amceba is a mass of simple protoplasm, very 

 similar to the inside material in a paramecium, and to that 

 inside the cells of higher animals. The animal has long been 

 a favorite with biologists because it shows the appearance 

 of protoplasm, which cannot be seen living in the opaque 

 cells of higher animals. 



272. Economic Relations of Amoebas. There are many 

 species of amcebas, and most of them are not known to be of 

 any special economic interest; but one species common in 

 tropical countries produces a serious form of intestinal disease 

 known as dysentery. 



273. Allies of Amoeba and Paramecium : Protozoa. 

 These animals are members of the lowest division of the 

 animal kingdom, a group of animals which are characterized 



