Chapter 22 

 Subclass 2 Actinopoda Calkins 



THE Actinopoda are divided into two orders as follows: 

 Without central capsule Order 1 Heliozoa 



With central capsule Order 2 Radiolaria (p. 417) 



Order 1 Heliozoa Haeckel 



The Heliozoa are, as a rule, spherical in form with many radi- 

 ating axopodia. The cytoplasm is differentiated, distinctly in Ac- 

 tinosphaerium, or indistinctly in other species, into the coarsely 

 vacuolated ectoplasm and the less transparent and vacuolated 

 endoplasm. The food of Heliozoa consists of living Protozoa or 

 Protophyta; thus their mode of obtaining nourishment is holozoic. 

 A large organism may sometimes be captured by a group of Heliozoa 

 which gather around the prey. When an active ciliate or a small roti- 

 fer comes in contact with an axopodium, it seems to become suddenly 

 paralyzed and, therefore, it has been suggested that the pseudopodia 

 contain some poisonous substances. The axial filaments of the axo- 

 podia disappear and the pseudopodia become enlarged and surround 

 the food completely. Then the food matter is carried into the main 

 part of the body and is digested. The ectoplasm contains several 

 contractile vacuoles and numerous refractile granules which are 

 scattered throughout. The endoplasm is denser and usually devoid 

 of granules. In the axopodium, the cytoplasm undergoes streaming 

 movements. The hyaline and homogeneous axial filament runs 

 straight through both the ectoplasm and the endoplasm, and ter- 

 minates in a point just outside the nuclear membrane. When the 

 pseudopodium is withdrawn, its axial filament disappears com- 

 pletely, though the latter sometimes disappears without the with- 

 drawal of the pseudopodium itself. In Acanthocystis the nucleus is 

 eccentric (Fig. 192, b), but there is a central granule, or centroplast, 

 in the center of the body from which radiate the axial filaments of 

 the axopodia. In multinucleate Actinosphaerium, the axilia filaments 

 terminate at the periphery of the endoplasm. In Camptonema, an 

 axial filament arises from each of the nuclei (Fig. 190, d). 



The skeletal structure of the Heliozoa varies among different 

 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 dif- 



406 



