6S 



THE INVERTEBRATA 



reproduction. In the Polythalamia binary fission does not occur, and 

 in some cases, 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 



att. ?fi- 



g. vac. 



nu. -r>«V^,^, 



Fig. 59. Arcella discoides, x 500. From Leidy. A, Seen from above. B, Seen 

 from the side, optical section, sh. shell; ^5. pseudopodia; op. edge of opening 

 into shell ; att. thread attaching animal to inner surface of shell ; nu. nucleus ; 

 f.vac. food vacuole ; g.vac. gas vacuole. 



nuclei and a chromidium present. Gas vacuoles in the protoplasm 

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

 Reproduction by binary fission, or by budding to form amoebulae 

 with fine pseudopodia (Nucleariae). In fresh waters. 



Difflugia (Fig. 60). Shell of sand grains, etc., united by organic 

 secretion, pear- or vase-shaped; pseudopodia lobose; one or two 

 nuclei and a chromidium present. Gas vacuoles sometimes formed. 

 In fresh waters. 



Euglypha (Figs. 7, 58). Shell resembling that of Difflugia but 



