198 DESCRIPTION OF [Polj/cfastrica. 



This family, along with the ArcelUna, form a very natural group 

 of Infusorial beings, especially, characterized by their pulpy con- 

 sistence, and variable expansions, serving the purposes of locomotion, 

 and probably of prehension. This group has pretty generally re- 

 ceived the appellation of Rhizopodes ; a term derived from their root- 

 like processes or feet. The extent of signification however, of the 

 name, has, unfortunately, not hitherto been sufiiciently determined 

 ujion : thus, Siebold (Lehrbuch der Vergleichenden, Anatomic, der 

 "Wii-bellosen, Thiere. Von. c. Th. N. Siebold. Berlin, 1848. p. 11.) 

 uses it for aU the beings in the families, Amwboea and Arcellma ; 

 whilst Dujardin restricts its use to the genera, included in the family 

 ArcelUna, (Ehr.) with some others, but excludes the Amoeha. 



The term Rhizopoda is useful to define the entire group, which 

 may indeed be called a class. (See Page 58.) 



Speaking of these pecuKar animalcules, Siebold observes, that 

 little is known of their internal structure, but they appear allied to 

 the Infusoria. Their bodies may be compared to a simple cell, 

 containing in their Parenchyma, a firm nucleous body, analagous to 

 that of the Infusoria, no special organs can be distinguished unlike 

 the true Infusoria. The Rhizopoda have no fixed form, no ciliated 

 surfaces, but are moved slowly onwards by ramifying processes, 

 protruded at various parts of the body, and continuous in substance 

 with it. 



Some of the Rhizopoda of Dujardin, are considered by Ehrenberg, 

 to belong, not to the Infusoria, but rather to another order of beings, 

 called by him Polythalamia, and by M. A. D'Orbigny, Foraminifera. 

 The main difference, separating the Rhizopoda from the Polythalamia, 

 is, according to Ehrenberg, the calcareous composition of the shells 

 of the latter, and the siliceous constitution of the Foraminifera. 

 Another general distinction between the two classes is, that whereas 

 in Polythalamia the variable and gelatinous processes protrude through 

 numerous regularly disposed holes, (foramina) in the shell, those of 

 the Rhizopoda, on the contrary, escape from a single opening of 

 greater dimensions. 



The following account of the habits and appearance of the Amoiba, 

 given by Dujardin, convey a clear notion of those animalcules. 



We may, in the first instance, perceive, on the glass slide, (under 



