THE MICROSCOPE AS A RECREATION 



127 



Fig. 115.— Difflugia. 



somewhat darker and harder material, is always present and is 

 essential to life. What part it plays is unknown ; it appears 

 to be a kind of vital spark, and is called the Nucleus. The other 

 organ is nothing more or less than a good-sized bubble, called 

 the Vacuole. 



The Amoeba, the simplest form 

 of animal that exists, is so colour- 

 less and so transparent that every- 

 thing going on in its interior is 

 visible. Its structure can be 

 imderstood at a glance, and start- 

 ing from this simple form we can 

 find creatures varying from each 

 other but slightly, which show 

 step by step an almost com- 

 plete series of stages of development up to elaborate organisms. 

 For instance, there is one species of Amoeba which has one 

 end of its body hardened into an unchanging shape — just one 

 corner only around which some of the jeUy has hardened up at the 

 edge, showing the commencement of the development of a 

 covering, while the rest of the creature is exactly like its simpler 

 brother, having, with the exception of this little corner, no fixed 

 shape, but pouring about as before. 



The next shape is reached in the Difflugia. It is an Amoeba 

 and possesses the same curious means of engulfing food ; but when 

 in the course of its meals it gets outside pieces of sand or similar 

 indigestible material, it retains them, fixing them around the surface 

 of its body until a cap is formed and only a small portion of the 

 jeUy is left free. These particles are cemented together with 

 some of the hardened jelly, and form a rough sheU in the shape of 

 an egg with one end broken ofi. From this open end the creature 

 flows in irregular projections of jelly to catch food, and crawls 



about carrying the shell on its back. 

 It seems to have a power of selection 

 as to the size and shape of the grains 

 that will form a satisfactory shell, 

 and, although there is not a perfect 

 regularity in its construction, it is 

 evidently not left entirely to chance. 

 A further development in the 

 direction of producing a protective 

 covering is shown in the beautiful 

 Heliozoa. In this case a spherical 

 shell is deposited, perforated with 

 tiny holes, through which fine rays of jelly exude in the 

 form of delicate filaments. Here the shell is not built up 

 of pieces of sand, but is probably formed of the products of 

 digestion. 



Fia. 116. — ^HeUozoa. 



