198 PROTOZOA. 



poiynucleate protoplasm divides into numerous uninucleate por- 

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

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

 species (Polystomella crispa, Hyalopus dujardinii, Miliolidae, etc.) 

 there is apparently a dimorphic alternation of generations. A 

 megalospheric generation (characterized by the long persistence of 

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

 produces zoospores, These develop into the microsphgeric genera- 

 tion (early poiynucleate, 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 plates (fig. 128); Diffiugia, 

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

 shelled Amoeba 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. Euglypha, 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 

 bottom in such numbers that a gram of the sand 

 50,000 of them. Thick beds of rock like 



FIG Difflu ia 



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



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

 diameter of about 1 mm. Rarely 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. Miliola (fig. 130). 

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

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

 bulloides (fig. 129), pelagic. Among the fossils the Nummulites need men- 

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

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



Order VI. Mycetozoa. 



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

 mals, by others as plants under the older name Myxomycetes 

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

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

 tion resembling that of many fungi. The plasmodia appear in 



