I. RHIZOPODA: RADIOLARIA 175 



centre of the body. Here lies the centrosome separated from the nucleus 

 (cf. p. 59), which controls the division of nucleus and protoplasm. The body 

 consists of cortical and medullary portions (fig. 121), distinguished by differences 

 in the protoplasm, but not separated by membrane. In the cortex are the con- 

 tractile vacuoles (cv); the medulla contains the usually single nucleus. Repro- 

 duction takes place by division, and one or both moieties may become swarm 

 spores, i.e., assume an oval form bearingatone poleoneortwoflagella(fig. 122, C). 

 These become widely distributed by the llagella before they resume the spherical 

 shape, lose their flagella and form pseudopvxlia. It frequently occurs that 

 several Heliozoa of the same species become connected by protoplasmic bridges, 

 and so form unions of from two to ten individuals (plasmogamy). True fertili- 

 zation (karyogamy) has only been observed in Actlnophrys and Acthwsphcerium. 

 Forms with skeleton and those without arc distinguished. Among those 

 with a silicious skeleton are Clathndina elegans, with a spherical lattice- work 

 skeleton supported on a stalk (fig. 122), and Acanthocystis lurfacca, skeleton of 

 radial, branching needles. To the forms without a skeleton belongs Actino- 

 sphccrium eichhorni (fig. 121), as large as a pinhead, milk-white, protoplasm 

 foamy from the numerous vacuoles, the different sizes of which distinguish the 

 cortical from the medullary parts. There are contractile vacuoles in the cortex, 

 many nuclei in the medulla. In encystment the foamy appearance is lost and 

 most of the nuclei are dissolved (fig. 123). Inside the gelatinous cyst the con- 

 tents are divided into primary cysts, corresponding in number to the persistent 

 nuclei, and these become enclosed in a silicious envelope (fig. 122, II). Each 

 primary cyst divides mitotically into two daughter cysts (III, i, 2), which become 

 mature by forming two polar globules (III, 3, 4) and then completely fuse with 

 each other (III, 5). The germ cells arising in this autogamic manner enclose 

 themselves in a resistent envelope (IV, 1-4), from which, after a considerable 

 period of rest, the young Actinospluzria escape (IV, 5). 



Order IV. Radiolaria. 



The Radiolaria, the most beautiful and most highly organized of the Rhizo- 

 poda, recall the Heliozoa. They are spherical, only rarely flattened into disks, 

 or taking conical or lobular shapes. A second resemblance lies in the delicate 

 pseudopodia, often with an axial filament. The most characteristic feature is 

 the central capsule. This is the central portion of the body surrounded by a 

 membrane, outside of which is the extracapsulnm. If the central capsule be 

 removed from the extracapsulum it not only lives but regenerates the lost parts, 

 while the extracapsular portions die. Since the protoplasm of both parts is 

 identical, the difference in regenerative powers can only depend on the nuclei, 

 which are confined to the central capsule. The capsular membrane is either 

 perforated on all sides by numerous pore canals or by openings in certain pla< 

 The protoplasm passes through these and spreads itself in the exiracapsulum. 

 The chief part of this is a gelatinous mantle, through which the protoplasm 

 extends as a fine network before it forms pseudopodia on the surface. In the 

 larger Radiolaria it may be extensive, containing vacuoles (extracapsular <j/r<W/) 

 developed in the protoplasmic net. 



The central capsule may be uni- or polynucleate. In the first case the 

 nucleus (fig. 124), a vesicle of appreciable size, lies in the centre of the capsule, 

 in the others the capsule is thronged by hundreds of small homogeneous nuclei. 

 The fact that certain species are nearly always multinucieate, others uniini- 

 cleate is due to the fact that in them the primitively single nucleus di\ide^ early, 

 while in the other it becomes considerably enlarged and forms in its interior 

 material for the secondary nuclei, which escape, the remainder of the nucleus 

 degenerating. 



