212 The Sarcodina 



and the protoplasmic core disappears, leaving a tubular mature stalk 

 attached only to the test (144). 



Although little is known about the life-cycles, fission within the test, 

 the development of a flagellate stage from one of the daughter organisms, 

 and encystment have been described (Fig. 5. 7) in Monomastigocystis 

 (129) and Hedriocystis (54). 



The taxonomic relationships of the Desmothoracina are still debatable. 

 Superficially, they show striking resemblances to typical Heliozoida. Al- 

 though the granular pseudopodia seem to be axopodia, they are some- 

 times so slender that the presence of axonemes is uncertain. The nucleus 

 is central in some species and eccentric in others, but no central granule 

 has been demonstrated. In view of the apparent absence of axonemes 

 and a central granule, Valkanov (148) suggested transfer of the Desmo- 

 thoracina to the Foraminiferida as another monothalamous group. 



The following genera have been assigned to the suborder: Choaiiocystis Penard (108; 

 Fig. 5. 8, B); ClathruUna Cienkowski (83, 108, 144; Fig. 5. 7, M); Blaster Grimm (108; 

 Fig. 5. 8, D); Hedriocystis Hertwig and Lesser (54, 108, 144; Fig. 5. 7, G-L); Monomasti- 

 gocystis de Saedeleer (129; Fig. 5. 7, A-F). 



Order 3. Radiolarida 



These marine organisms, with a geological history dating at least 

 from Lower Silurian and probably from Cambrian time, are apparently 

 the oldest known group of animals. Their most striking feature is their 

 skeleton, which has undergone specialization to a remarkable degree. The 

 general organization of the body and the possession of axopodia relate 

 them to the Heliozoida, but the central capsule, separating inner and 

 outer zones of protoplasm, is a differential feature. 



The central capsule is nearly always a distinct layer, usually single 

 but sometimes double (Fig. 5. 13, A), and can be detected without diffi- 

 culty except in a few Actipylina (Acantharina). The capsule may be 

 spherical, ovoid, or sometimes lobate or branched (Fig. 5. 11, C), and is 

 composed of organic material designated variously as chitin, pseudochitin, 

 or tectin. The capsule may be resorbed more or less completely in fission 

 of the simpler species, it may increase in diameter with growth of the 

 organism, and it may be somewhat changeable in form even in the mature 

 organism. Perforations, either distributed uniformly or concentrated in 

 one or more groups, permit cytoplasmic continuity and also serve as 

 taxonomic features. 



The skeleton of the Actipylina may be composed largely of strontium 

 sulphate, usually with a radial arrangement of the skeletal elements. The 

 basic components are spines which extend radially from the center of the 

 body, passing through the central capsule (Fig. 5. 9, A). At the surface of 

 the body there may also be a lattice-work test, or shell, which is fused with 

 the radial spines (Fig. 5. 9, D). For the other groups of Radiolarida, silice- 



