72 THE INVERTEBRATA 



« 

 some and in certain other features of the nucleus (Fig. 55), and in 



forming only four, instead of eight, nuclei in the cyst. This species 



breaks up by digestion cells of the intestinal epithelium and other 



tissues, absorbs the soluble products, and ingests portions of the 



destroyed cells and also red corpuscles. 



Pelomyxa (Fig. 57). Large, multinucleate species, living in, and 



feeding by ingesting, the mud of stagnant fresh waters rich in vegetable 



debris. The cytoplasm contains glycogen granules (see p. 20). 



Order FORAMINIFERA 



Sarcodina w^hich have either a shell or reticulate pseudopodia or, 

 usually, both these features ; and in pelagic species a vacuolated outer 

 layer of protoplasm. 



The shell may be of one or of several chambers, and is composed in 

 different cases of different materials, nitrogenous, calcareous, siliceous, 

 or of foreign particles. 



The pseudopodia may be lobose, filose, reticulate without streaming 

 of particles along them, or reticulate with streaming. The latter type 

 alone is found in the Polythalamia. 



The reproduction of the single-chambered forms (Monothalamia) 

 is both by binary and by multiple fission. In binary fission, Lie- 

 herkUhnia and Trichosphaerium divide the shell. In the rest, a portion 

 of the protoplasm emerges from the old shell and secretes a new one 

 (Fig. 58), the nucleus or nuclei divide, one of the products of each 

 passing into the protruded protoplasm while the other remains in the 

 old shell, and the two portions of protoplasm break apart. Multiple 

 fission usually produces amoebulae, sometimes flagellulae. The latter 

 are known or suspected to be gametes. In these forms there does not 

 usually appear to be a regular alternation of sexual and asexual repro- 

 duction. In the Polythalamia binary fission does not occur, and in 

 some of them, perhaps in all, there is a more or less regularly alternate 

 production of asexual amoebulae and flagellate gametes. 



Suborder MONOTHALAMIA 



Foraminifera, usually of freshwater habitat; with non-calcareous, 

 single-chambered shells; whose pseudopodia are rarely reticulate; 

 and whose protoplasm does not extend as a layer over the shells. 



Arcella (Figs. 22, 59). Shell pseudochitinous, shaped like a tam-o'- 

 shanter cap, finely sculptured; pseudopodia lobose; two or several 

 nuclei and a chromidium present. Gas vacuoles in the protoplasm 

 are said to contain oxygen and to have a hydrostatic function. Re- 

 production by binary fission, or by budding to form amoebulae with 

 fine pseudopodia {Nucleariae). In fresh waters. 



