HELIOZOA 



29 



-/u 



its assistance. The pseudopodia then shorten and thicken and the food- 

 particle is drawn down to the surface and finally into a food-vacuole, 

 the ectoplasm closing over it. As the pseudopodium thickens and 

 shortens the axial filament becomes resolved into ordinary fluid cytoplasm. 

 It is therefore not a permanent structure but merely cytoplasm — possibly 

 an extension of the endoplasm — temporarily modified for a special 

 function. 



The food-vacuoles pass down into the endoplasm where the digestive 

 processes take place (Fig. rijf.v). In the endoplasm is also contained 

 the nuclear apparatus which consists not of a single nucleus but of a 

 large number • — several 

 hundred there may be 

 in a large specimen. 

 The Actinosphaerium is 

 therefore a syncytium 

 rather than a cell. 



The life -history of 

 Actinosphaerium shows 

 some important features. 

 Under ordinary circum- 

 stances the Actino- 

 sphaerium reproduces 

 by a simple process of 

 fission. At certain 

 periods however, occa- 

 sioned apparently by the 

 onset of unfavourable 

 conditions, a much more 

 complicated process of 



reproduction, of a sexual type, takes place. The pseudopodia are 

 withdrawn, the axial filaments disappearing, and the Actino- 

 sphaerium surrounds itself with a thick protective jelly-like cyst. 

 The vacuoles practically disappear, and the protoplasm becomes uni- 

 formly granular and opaque, laden with minute particles some of which 

 are apparently reserve food-material while others are particles of silica, 

 afterwards used to strengthen the zygote cyst. About nineteen out of 

 every twenty nuclei disappear and the protoplasm segregates round the 

 remaining nuclei to form rounded uninucleate masses which are termed 

 the primary cystospores. Each of these secretes a special jelly-like cyst 

 and then within this divides with mitosis into two secondary cystospores. 

 The nucleus of each of these undergoes mitosis without however this 



Actinosphaerium, c.v. Contractile vacuole ; ect, ectoplasm ; 

 f.Vj food-vacuole ; «, nucleus. 



