112 MICROSCOPY FOR BEGINNERS. 



construction. Thus the members of one genus, Difflu- 

 gia, build themselves shells of sand grains cemented 

 together with the most perfect regularity, every grain 

 exactly fitting to its place. Yet, when the young Dif- 

 flugia happens to be where suitable sand is scarce, it 

 will build its shell of diatoms, often using those that 

 are longer than the completed covering, attaching them 

 lengthwise, side by side, and parallel to each other. An- 

 other genus, Arcella, secretes from its body a brown 

 shell of delicate membrane which, with a high power, 

 is seen to be formed in minute hexagons. And still 

 another, Clathrulina, the most beautiful of all the fresh- 

 water Ehizopods, lifts itself on a long stem, and there 

 surrounds its body by a hollow latticed sphere, and 

 through the openings in the walls extends its pseudo- 

 podal rays in search of food. 



In the unprotected forms those without a shell the 

 pseudopodia are protruded from any part of the body ; 

 in those preparing shells they are protruded from that 

 portion of the body immediately in contact with the 

 mouth of the shell, through which they often extend for 

 a long distance as very fine, branching threads. "With a 

 few exceptions the bodies of the Rhizopods are colorless ; 

 in those exceptions the coloration is usually due to the 

 presence of colored food, and so is diffused throughout 

 the entire protoplasm, or it is confined to the parts near 

 the surface, the central portion being nearly colorless. 

 The pseudopodia are never colored. 



Not only do the Rhizopods move by means of these 



