30 PROTOZOA fCH. 



comparatively common inhabitants of our ditches. The first is, 

 however, exceedingly minute, not more than TT yVo tn f an i ncn 

 in diameter and possesses only a single nucleus, whereas the second 

 is large enough to be just visible to the naked eye, and has about 

 200 nuclei. There is here repetition of the nuclei, but no division 

 of the protoplasm, whereas in Polystomella there is segmentation of 

 the protoplasm, but no corresponding multiplication of the nuclei. 

 If both were to occur simultaneously and to correspond so that the 

 body were to consist of a number of segments of protoplasm, each 



FIG. 8. Actinophrys sol x about 800. From Bronn. 



1, Ectoplasm. 2. Endoplasm. 3. Contractile vacuole. 4. Food vacuole. 

 5. Nucleus. 6. Axis of a pseudopodium, stiffer than the protoplasm 



which covers it. 



with its nucleus, the animal would be said to be multicellular, 

 each unit being spoken of as a cell; but it would no longer be a 

 Protozoon. 



Actinosphaerium can retract its pseudopodia and encyst itself. 

 The cyst wall contains numerous spicules of silica. This cyst is 

 called the mother-cyst. The nuclei are now rapidly absorbed till 

 only a few of the original 200 remain. The protoplasm then divides 

 into masses each with one of these residual nuclei. These masses 

 secrete round themselves the primary daughter-cysts. The 

 protoplasm inside each daughter cyst divides into two spores which 

 secrete round themselves secondary cysts. The nucleus of each 



