162 UNICELLULAR ANIMALS. 



Structure. Lying in the entoplasm, usually near the pos- 

 terior extremity, is a nucleus (w, Fig. 84), having the form of 

 a bi-concave disk and largely made up of coarse granules of 

 chromatin (cf. p. 23). Amoeba is therefore at once a single 

 cell and a unicellular organism, morphologically equivalent to a 

 single tissue-cell of a higher animal or to the germ-cell from 

 which every multicellular form arises. The body of Amoeba is 

 a one-celled body. 



The protoplasm (cytoplasm) consists of a clear basis, and (in 

 the case of the entoplasm) of innumerable granules extremely 

 diverse in form and size, and frequently differing in character 

 in different individuals. Often they are in the form of rhom- 

 boidal crystalline bodies; in other cases they are rounded or 

 irregular. Their precise chemical composition is uncertain, but 

 they are probably complex organic compounds, a product of 

 metabolism and serving as reserve food-matter.* 



Vacuoles. The protoplasm often contains rounded vacuoles 

 of which the three following kinds may be distinguished : 



(a) Water-vacuoles (w.v. Figs. 84, 85), filled with water, 

 lying in the entoplasm and carried along in its currents. 



(b) Food-vacuoles (f.v\ also lying in the entoplasm, con- 

 taining the solid food-matters that have been ingulfed. Within 

 them digestion takes place. When this process is completed 

 they approach the exterior usually at some point near the 

 posterior end the outer wall breaks through, and the innutri- 

 tions remnants are cast out, the ectoplasm closing up the breach 

 immediately afterwards. Thus Amceba has no mouth, ali- 

 mentary canal, or anus, but the general mass of protoplasm 

 plays the role of all three. 



(c) Contractile vacuole (c.v). Usually single, sometimes 

 double, lying near the posterior end, and filled with liquid. 

 This is sharply distinguished from the other vacuoles by its 

 rhythmical pulsation, expanding (diastole') and contracting (sys- 

 tole) at regular intervals. During the diastole the vacuole slowly 

 fills with liquid which drains into it from the surrounding proto- 

 plasm. At the systole, which is very sudden, this liquid is forci- 

 bly expelled to the exterior through an opening that breaks 



* In some species of Amceba the entoplasm may also contain innumerable 

 grains of sand taken in from the exterior, but this is not the casein A. Proteus. 



