RHIZOPODA 733 



is undergoing digestion during its stationary period. No encysting 

 process has been noticed in Lieberkuehnia ; but Cienkowsky has dis- 

 covered that in L. paludosa reproduction is effected by a process of 

 fission, which commences with the formation of a new pseudopodial 

 stalk at the base of the animal, the envelope being perforated at this 

 point. As the marine type of it occurs on our own coasts, the fresh- 

 water type may very likely be found in our ponds, and either may 

 be recommended as a most worthy object of careful study. 



> 

 RHIZOPODA. 



We now arrive at the group of rhizopods, or ' root-footed ' animals, 

 first established by Dujardin for the reception of the Amoeba and its 

 allies, which had been included by Professor Ehrenberg among his 

 iiifusory animalcules, but which Dujardin separated from them as 

 being mere particles of sarcode (protoplasm), having neither the defi- 

 nite body- wall nor the special mouth of the true Infusoria, but put- 

 ting forth extensions of their sarcodic substance, which he termed 

 pseudopodia (or false feet), serving alike as instruments of locomotion 

 and as prehensile organs for obtaining food. According to Dujardin's 

 definition of this group, the Monerozoa, already described, would be 

 included in it ; but it seems on various grounds desirable to limit the 

 term Rhizopoda to those Protozoa in which the presence of a nucleus, 

 the differentiation of an ectosarc (or firmer superficial layer of proto- 

 plasm) from the semi-fluid endosarc, together with the more definite 

 form and restricted size, indicate a distinct approach to the condition 

 of true cells. Many different schemes for the classification of the 

 rhizopods have been proposed, but none of them can be regarded 

 as entirely satisfactory, our knowledge of the reproductive processes, 

 and of other important parts of the life-history of these creatures, 

 being still extremely imperfect ; and as some parts of the scheme 

 proposed by the Author many years ago, 1 based on the characters of 

 the pseudopodial extensions, have been accepted by more recent 

 systematists, it seems best still to adhere to it. 



I. In the first division, Reticularia, the pseudopodia freely 

 ramify and inosculate, so as to form a network, exactly as in Lieber- 

 kuehnia, from which they are distinguished by the possession of a 

 nucleus and by the investment of their sarcodic bodies in a firm 

 envelope. This is most commonly either a calcareous shell of very 

 definite shape, or a test built up of sand- grains or other minute 

 particles more or less firmly united by a calcareous cement exuded 

 from the sarcodic body. These testaceous forms, which are exclu- 

 sively marine, constitute the group of Foraminifera, whose special 

 interest to the microscopist entitles it to separate consideration ; 

 and it is only for convenience that two Reticularia which in- 

 habit fresh water also, and the envelopes of whose bodies are 

 usually membranous, are here separated from the Foraminifera (to 

 which they properly belong) for description as types of the group. 

 The Reticularia have little locomotive power, and only seem to 



1 Natural History Review, 1861, p. 456; and Introduction to the Study of the 

 Foraminifera, 1862, chap. ii. 



