140 HIDDEN BEAUTIES OF NATURE 



organisms are, as we have already said, provided 

 with a shell or home of hard material. In the 

 majority of instances the shell consists of hardened 

 parts of the animal substance, a lomething which 

 pertains to the inherent structure of the animal itself. 

 The shell is then said to be intrinsic. 



But frequently we find rhizopods possessing a 

 power that enables them to appropriate flinty 

 particles, transparent quartz sand, diatom cases, and 

 sponge spicules, and to collect and fashion into 

 artistic pellets the clay in the surrounding water 

 with which to make their shells or cases. In such 

 cases, inasmuch as all the materials are collected 

 from external sources, the shells are said to be 

 extrinsic. 



It will be well to thoroughly grasp these two 

 definitions, because they form the basis of all our 

 knowledge of rhizopods, whether of fresh or of sea 

 water, and it is in connection with the shells that we 

 shall observe the hidden beauty we are seeking. 

 There is scarcely a class of animals in the whole 

 range of the animal kingdom about which such 

 divergence of opinion has been held. 



Dozens of naturalists have studied them in the 

 endeavour to arrange them into species and genera, 

 but the absolute distinctions which appear more 

 definitely to characterise the higher forms of life are 

 absent in the rhizopods. The members of this class 

 of life are infinitely variable, and therefore impossible 

 to classify completely. 



