XXIV THE VOYAGE OF H.M.S. CHALLENGER. 



the great majority of Ph.eodaria the spheroidal central capsule also possesses a pair of 

 parapylae near the opposite apical pole of the main axis (Tripylea), and these determine (as 

 the right and left secondary openings) an isopolar frontal axis. Hence, strictly speaking, 

 in most Ph^odaria the central capsule has the geometrical ground-form of the amphi- 

 thect pyramid (as in the Ctenophora), with an allopolar vertical main axis, and two 

 unequal, but isopolar, horizontal transverse axes. In many Ph.eodaria the skeleton also 

 has this amphithect pyramidal ground-form, e.g. , the bivalved PhjBoconchia and part 

 of the P h 86 o g r m i a. On the contrary, in the rest of the PfliEODARTA the skeleton 

 exhibits very various geometrical ground-forms, independent of that of the central capsule. 

 In the Phseosphseria it forms preferably spheres or endospherical polyhedra, as also 

 in the Castanellida and Circoporida among the Phseogromia; among the Circoporida 

 there are also seen with remarkable distinctness the regular polyhedra (especially the 

 dodecahedron and icosahedi'on). Isopolar mouaxonia are found among the Aulosphterida 

 (Aulatractus) and Orosphserida ; allopolar monaxonia among the Challengerida {Litho- 

 gromia). The Medusettida and Tuscarorida show various forms of regular pyramids 

 (aUopolar stauraxonia) ; and finally, the Challengerida are for the most part centroplanar 

 or bilateral. Thus the Ph.eodaria present a great wealth of difi"erent geometrical ground- 

 forms in the development of their skeleton, not in that of their central capsule. 



Chapter II.— THE CENTRAL CAPSULE. 



5L Components of the Central Capsule. — In all Eadiolaria without exception, at 

 some period of life or other, the central portion of the soft body is separated from the 

 peripheral portion by an independent, anatomically recognisable membrane ; this mem- 

 brane with all its contents is designated the central capsule, and is the peculiar central 

 organ of the unicellular body, which distinguishes the Eadiolaria most clearly from 

 the other Rhizopoda. In the great majority of the Radiolaria the volume of the central 

 capsule is less than that of the surrounding peripheral soft body which we place in 

 opposition to it as " extracapsulum." The " capsule-membrane," which separates these 

 two constituents, arises very early in most Radiolaria, and persists throughout their 

 whole life. In some species, however, the membrane only appears later, immediately 

 before the formation of the spores, and hence is absent for a considerable j)eriod. Re- 

 garded as a whole, then, the capsule consists of the following j)arts : — (1) the capsule- 

 membrane ; (2) the enclosed endoplasm, or intracapsular protoplasm ; (3) the nucleus. 

 But in addition, many other non-essential structures may be enclosed in the central 

 capsule, especially hyaline spheres (vacuoles), fatty spheres, pigment granules, crystals, &c. 



The central capsule was first described in my Monograph in 1862 (pp. 69—82) as the most 

 characteristic component of the Eadiolarian organism, and distinguished from the whole extracapsular 



