II 



PHYLUM PROTOZOA 



49 



ways (F, II), und the pseudopods may be prolongations of con- 

 siderable relative size (C), or mere wave-like elevations of the 

 surface (6r). Sometimes specimens are found in which neither 



- ; 



FIG. 31. Frotamoeba primitiva. Sliowing chauges of form and three stages in binary 

 fission. (After Haeckel, from Parker's Biology.) 



nucleus nor vacuole is present ; these are placed in the genus 

 Protamceba (Fig. 31). This and other non-nucleate forms possess a 

 potential nucleus in the form of minute scattered granules of chro- 

 matin (chromidia). 



The largest of the naked or shell-less Lobosa is Pelomyxa, which 

 may be as much as 8 mm. in diameter ; it is multi-nucleate and is 

 further distinguished by the presence of numerous non-contractile 

 vacuoles in the endosarc. 



Skeleton. We may 

 understand the relation 

 of the shelled to the 

 shell-less Lobosa by 

 supposing an Amoeba to 

 draw in the pseudopods 

 from the greater part of 

 its body, and to secrete, 

 from that part only, a 

 cell- wall ; such a cell-wall 

 or capsule would differ 

 from a cyst in having 

 an aperture at one end 

 to allow of the protrusion 

 of pseudopods from a 

 small naked area. This 

 is exactly what we find 

 in Arcella and its allies 

 (Fig. 32, A-C), in which 

 the shell is chitinoid. 

 A different kind of shell 

 is found in Difflugia (D), 

 which secretes a gela- 

 tinous coating to which 

 minute sand-grains and other foreign particles become attached. 



The prevailing mode of multiplication is by means of binary 

 fission. But multiple fission also occurs. In such a case the animal 

 passes into the encysted condition, and nucleus and protoplasm 



VOL. i E 



FIG. 32. A, Quadrula symmetrica ; B, Hyalo- 

 sphenia lata ; C, Arcella vulgaris ; 7), Difflugia 

 pyriformis. (From Lang's Comparative Anatomy.) 



