6 FRESH- WATEB EHIZOPODS OF NORTH AMERICA. 



delicacy, of coarser finger -like processes, or of rounded lobes. They often 

 branch and assume a more or less root-like appearance, whence Dujardin 

 gave the class the name of Rhizopods. 



As previously intimated, the simplest kinds of Rhizopods are unpro- 

 vided with hard parts, or even a membranous investment, and they present 

 to the outside medium in which they live nothing but the naked mass of 

 protoplasm of which they consist. By far the greater number of the class 

 are protected and supported by some kind of skeleton of hard material, 

 consisting of spicules or a trellis-work of silex, or a shell of chitinoid 

 membrane, of limestone, or of the former material. Mostly the hard part is 

 intrinsic, or pertains to the inherent structure of the animal, but frequently 

 is also more or less extrinsic. In the latter instances, the shell usually 

 consists of siliceous particles, commonly hyaline quartz sand, diatom cases, 

 and sponge spicules. 



Besides some general differences in the character of the soft parts, the 

 sustaining skeleton, or protective shell, of the Rhizopods, exhibits a great 

 variety in form, construction, and arrangement of structure ; frequently is 

 highly intricate and often remarkable for beauty of apparent design. 



On the general differences observed in the soft body-mass and its 

 pseudopods, and on the absence or presence of hard parts with their form 

 and constitution, the ordinal and other subdivisions of the Rhizopods are 

 founded. It however appears from the researches, especially of British 

 authorities, such as Carpenter, Williamson, Wallich, Brady, Parker, and 

 Jones, that the members of the class are infinitely variable, and that indeed 

 no absolute distinctions of species and genera exist, such as appear more 

 definitely to characterize the higher forms of animal life. My own investi- 

 gations rather confirm this view, and, under the circumstances, we can only 

 regard the more conspicuous and prevailing forms as so many nominal 

 species, in likeness with the species of higher organic forms, more or less 

 intimately related, and by intermediate forms or varieties merging into one 

 another. 



As is the case with all other groups of organic beings, few authorities 

 agree in the classification of the Rhizopods ; and it is for convenience 

 rather than from studied opinion that I have adopted the following ordinal 

 arrangement 



