246 HANDBOOK OF PROTOZOOLOGY 



When the pseudopodium is withdrawn, its axial filament dis- 

 appears completely. The latter sometimes disappears without 

 the withdrawal of the pseudopodium itself. 



In Acanthocystis the nucleus is eccentric (Fig. 100, b). 

 There is, however, a central granule, or centroplast, in the 

 center of the body from which radiate the axial filaments of 

 the axopodia. In multinucleate Actinosphaerium, the axial 

 filaments terminate at the periphery of the endoplasm. In 

 Camptonema, an axial filament arises from each of the numer- 

 ous nuclei (Fig. 98, c). 



The skeletal structure of the Heliozoa varies among dif- 

 ferent species. The body may be naked, covered by a gelatinous 

 mantle, or provided with a lattice-test with or without spicules. 

 The spicules are variable in form and location and may be used 

 for specific difterentiation. In some forms there occur colored 

 bodies bearing chromatophores, which are considered as 

 holophytic Mastigophora living in the Heliozoa as symbionts. 



The Heliozoa multiply by binary fission or budding. In- 

 complete division may result in the formation of colonies, as 

 in Rhaphidiophrys. In Actinosphaerium, mitotic division of 

 the nucleus has been observed by Hertwig, who described the 

 appearance of some 150 chromosomes, conspicuous centrosomes, 

 and spindle fibers. In Acanthocystis and Oxnerella (Fig. 19), 

 the central granule behaves somewhat like the centrosome in a 

 metazoan mitosis. Budding has been known in numerous 

 species. In Acanthocystis the nucleus undergoes amitosis 

 several times, thus forming several nuclei, one of which re- 

 mains in place while the others migrate toward the body sur- 

 face. Each peripheral nucleus becomes surrounded by a 

 protruding cytoplasmic body which becomes covered by 

 spicules and which is set free in the water as a bud. These 

 small individuals are supposed to grow into larger forms, the 

 central granule being produced from the nucleus during the 

 growth. Formation of swarmers is known in a few genera. 

 Sexual reproduction occurs in some forms (p. 57). 



The Heliozoa live chiefly in fresh water, although some in- 

 habit the sea. Only a few are attached forms. According to 

 Hertwig and Lesser, the Heliozoa are here divided into four 

 suborders: 



