THE HELIOZOA 19 



out help from the radial pseudopodia ; a larger creature is seized by 

 a group of radial pseudopodia, which converge round it, generally 

 (always ?) losing their axial filaments, and send out amoeboid 

 processes, which more or less completely engulf the prey. The 

 mass formed by these fused processes and the organism they con- 

 tain travels towards the body, where it meets and fuses with an 

 ingestive process. 



Actinophrys is capable of performing various rolling or creeping 

 movements on the bottom of the pond, but the creature spends 

 much of its time suspended in the water, where it has a certain 

 power of rising and of sinking, though the way in which this is 

 effected is altogether obscure. 



At intervals Actinophrys may withdraw its pseudopodia, the 

 axial filaments of which disappear ; it may then secrete a complex 

 cyst of two layers an outer, fairly thick transparent layer of gela- 

 tinous consistence, within which is a second, thinner layer. After 

 the formation of these layers, the vacuoles disappear, the contractile 

 vacuole being the last to go, and the whole body shrinks. The 

 nucleus now divides mitotically (cf. infra, pp. 25-27), and the cyst 

 divides into two, each of which becomes spherical. Within each 

 of the resulting cysts a third hard, opaque membrane is secreted, 

 and a period of quiescence ensues, after which the walls are ruptured 

 and the creature emerges, new pseudopodia being rapidly formed. 

 This account is based on that given by Schaudinn (17), who says 

 that each daughter -cyst may divide again before entering on a 

 period of quiescence ; on the other hand, many observers describe 

 a process of encystment which is not accompanied by any division 

 whatever. 



Just as encystment may occur without fission, so fission may, 

 according to Schaudinn, occur without encystment. An individual 

 about to divide in this way withdraws its pseudopodia, and a 

 peculiar mitosis takes place, not accompanied by disappearance of 

 the nuclear membrane or by the formation of centrosomata (infra, 

 p. 28) ; this is followed by fission of the cell-body, and pseudo- 

 podia are shortly afterwards emitted. 



The processes of fission just described, whether accompanied by 

 encystment or not, are asexual, since there is no previous fusion 

 of individuals or of nuclei. A process of plastogamic fusion, involv- 

 ing the union of a number of individuals (as a rule by the ectoplasm 

 only), without nuclear fusion, frequently occurs. The number 

 of individuals so united is frequently two ; but it may be over 

 thirty (Schaudinn). Plastogamic individuals lose their pseudo- 

 podia on the surfaces by which they are attached to each other 

 but retain them elsewhere, and the union is not necessarily followed 

 by a period of quiescence. Individuals which have been united in 

 this way for some time may separate without withdrawing those 



