66 



THE SMALLEST LIVING THINGS 



Fig. 33— AN ABORAL* VIEW OF THE SHELL OF A COMMON FRESHWATER 

 DINOPHYCEAE, CERATIUM TRIPOS 



Sutures show the limits of the shell-forming plates; the groove (annulus) is formed by a single 

 plate. Certain plates are drawn out into horns, two on the hypotheca (lower half), and two 



(fused) on the epitheca (upper half) 



After Stein 

 Magnification, 400 



many of them are naked, most of them are covered by a definite 

 membrane or test, mostly of cellulose but often impregnated with 

 inorganic, sometimes calcareous, deposits. The test may be sim- 

 ple and made up apparently of one piece, or it may have two 

 valves, or it may be composed of plates of definite form and ar- 

 rangement. Such plates are often drawn out into typical horns 



* Aboral (ab-6'ral), opposite to or away from the mouth. 



