198 PROTOZOA. 



polynucleate protoplasm divides into numerous uninucleate por- 

 tions ('embryos') which frequently, while still within the 

 mother, develojj a shell of one or several chambers. In several 

 species [Polystomella crispa, Hyalopns dujardinii, MiliolidEe, etc.) 

 there is apparently a dimorphic alternation of generations. A 

 megalospherio generation (characterized by the long persistence of 

 a large 'principal nucleus,' and often of a large central chamber) 

 produces zoospores. These develop into the microsplijeric genera- 

 tion (early polynucleate, often with small central chamber), which 

 form ' embryos ' (supra), which in turn become megalospheres. 



Sub Order I. MONOTHALAMIA. Mostly fresh-water forms. These 

 species never have calcareous shells, but shells of chitin or silica, often 

 strengthened by foreign bodies. Contractile vacuole usually present. 

 Pseudopodia either lobose or filiform, branched or simple. A. Forms 

 with finger-form pseudopodia: Arcella, brown disc-like shell, two or 

 several nuclei; Quadrula, shell of many square pjlates (fig. 128); Bifflugia, 

 (fig. 131 with shells of sand. These forms may be regarded as merely 

 shelled Amcebce and are frequently referred to the 

 Lobosa. All other Foraminifera are characterized 

 by the filiform branching or anastomosing pseudo- 

 podia. B. Forms with branching and anastomosing 

 filiform pseudopodia. Euglyplia, shell of oval plates; 

 Gromia (fig. 17), marine, shell a horny sac. 



Sub Order II. POLYTHALAMIA. Exclusively 

 ^, marine, living on aquatic plants, on the bottom or 

 pelagic. The shells, when not dissolved, fall to the 

 ' I ■" V ' , bottom in such numbers that a gram of the sand 



„ ,,,,,», mav contain 50,000 of them. Thick beds of rock like 



Fig ^il —Difffngia ^ ' 



(Orig.) the chalk, the nummulitic limestone, and the green- 



sand are largely foraminif eral in origin. The living species have an average 

 diameter of about 1 mm. Karely species have a diameter of several cen- 

 timeters {Psammonyx vulcanicus, 5-6 cm.). The fossil nummulites reach 

 a diameter of 6 cm. A. Shell wall massive, the terminal pseudopodal open- 

 ing being the only communication with the exterior. Miliohi (fig. 130). 

 B. Perforata. Shell perforated by many pores; the terminal opening may 

 be lacking. Polystomella, Rotalia (fig. 117), bottom dwelling; Glohigerina 

 hulloides (flg. 129), pelagic. Among the fossils the Nummulites need men- 

 tion as well as the Eozoon canadense of the extremely old Laurentian beds 

 of Canada, the animal nature of wliicli is denied by most students. 



Order VI. Mycetozoa. 



The Mycetozoa or slime animals are regarded bv some as ani- 

 mals, by others as plants under the older name Myxomycetes 

 (slime moulds). Tlie first position is supported by the structure 

 of the motile stage, the plasmodiitm, the second by the reproduc- 

 tion resembling that of many fungi. The plasmodia appear in 



