202 The Sarcodina 



to develop a test is widely distributed. Such structures are found in 

 Testacida and Foraminiferida and in the majority of Heliozoida. The 

 lattice-work skeletons of many Radiolarida are analogous developments. 



The Sarcodina as a group are widely distributed in fresh and salt 

 water and in the soil. However, the Radiolarida have remained marine 

 and the Foraminiferida which have invaded fresh water are primitive 

 types sometimes considered Testacida. A number of the Sarcodina are 

 parasitic. Various sessile forms may be epiphytic or epizooic, but endo- 

 parasitism is limited to the more primitive species or to possibly degen- 

 erate representatives of certain groups. 



On the basis of pseudopodial equipment, the Sarcodina are often di- 

 vided into two classes, Actinopodea and Rhizopodea. By definition, the 

 Actinopodea possess axopodia. The Rhizopodea may have any other kind 

 of pseudopodia but do not develop axopodia. 



CLASS 1. ACTINOPODEA 



These are mostly floating or sessile organisms, although flagellate 

 stages are known in a few genera. Accessory lobopodia are developed oc- 

 casionally, at least in certain species. The class may be divided into three 

 orders: (1) Helioflagellida, with one or more flagella as either a perma- 

 nent feature or a characteristic of the dominant phase in a dimorphic 

 cycle; (2) Heliozoida, in which a flagellate stage apparently is rare and 

 the inner cytoplasm is not separated from the outer zones by a central 

 capsule; (3) Radiolarida, in which a central capsule is characteristic and 

 skeletal structures are more highly developed than in Heliozoida. 



Order 1. Helioflagellida 



The relationships of this group are uncertain, and members of the 

 order have been classified as Rhizomastigida (Mastigophora) and Pro- 

 teomyxida, as well as Helioflagellida. The presence of axopodia, and also 

 a "central granule" in certain genera, suggests closer affinities with the 

 Heliozoida than with the Rhizomastigida or Proteomyxida. The Helio- 

 flagellida are of interest as possible sources of data bearing on phylogeny 

 of the Actinopodea. 



The following genera may be assigned to the order: Acinetactis Stokes (141, 143; 

 Fig. 5. 1, A); Actinomonas Kent (45; Fig. 5. 1, K); Ciliophrys Gruber (45; Fig. 5. 1, D, 

 E); Dimorpha Gruber (Fig. 5. 1, I-J); Dimorphella Valkanov (143; Fig. 5. 1, B, C); 

 and Tetradimorpha Hsiung (61; Fig. 5. 1, F-H). A "central granule," from which the 

 axoncmes of the axopodia radiate, has been demonstrated in Dimorpha, Dimorphella. 

 and Tetradimorpha. This central granule behaves as a centrosome during mitosis in 

 Dimorphella elegans (Fig. 5. 1, C). With the possible exception of Tetradimorpha, the 

 pseudopodia show the granules characteristic of axopodia; streaming of the granules 

 has been described in Acinetactis and Dimorphella. More or less complete retraction 

 of the pseudopodia occurs in swimming stages of Acinetactis, Ciliophrys, Dimorpha, 

 and Tetradimorpha. Both marine and fresh-water species of Ciliophrys have been 

 described; the other genera have been reported from fresh water. 



