EADIOLARIA.] PROTOZOA 851 PIG. XVI. Radiolaria. 1. Lithocircus annularis, Hertwig; one of the Monopylooa. Whole animal in the living state (optical section), a, nucleus ; b, wall of the central capsule ; c, yellow cells ; d, perforated area of the central capsule (Monopyltea). 2. Cystidium inenne, Hertwig ; one of the Monopyhca. Living animal. An example of a Monopylaeon destitute of skeleton, a, nucleus ; b, capsule-wall ; c, yellow cells in the extracapsular protoplasm. 3. Carpncanium diadema, Haeck. ; optical section of the bee hive-shaped shell to show the form and position of the protoplasmic body. a, the tri-lohed nucleus ; b, the siliceous shell ; c, oil-globules ; d, the per forate area (pore-plate) of the central capsule. 4. Ccelodendrum gracillimum, Haeck. ; living animal, complete ; one of the Tripylsea. a, the characteristic dark pigment (pheeodium) surrounding the central capsule b. The peculiar branched siliceous skeleton, consisting of hollow fibres, and the expanded pseudopodia are seen. 5. Central capsule of one of the Tripylcea, isolated, showing a, the nucleus ; b,c, the inner and the outer lamina; of the capsule-wall ; d, the chief or polar aperture ; e,c, the two secondary apertures. C, 7. Acanthometra Claparedei, Haeck. 7 shows the animal in optical section, so as to exhibit the characteristic meeting of the spines at the central point as in all Acanthometridea ; C shows the transition from the uninuclear to the multinuclear condition by the breaking up of the large nucleus, a, small nuclei ; 6, large fragments of the single nucleus ; c, wall of the central capsule ; d, extracapsular jelly (nut pr toplasm); e, peculiar intracapsular yellow cells. 8. Sponyo- sph;vra Streptacantha, Haeck. ; one of the Peripyhea. Siliceous skeleton not quite completely drawn on the right side, a, the spherical extra capsular shell (compare Fig. XIV. 17), supporting very large radial spines which are connected by a spongy network of siliceous fibres. 9. Aulo^ ili <:m elegantissfma, Haeck. ; one of the Phseodaria. Half of the spherical siliceous skeleton. capsule has a double membrane and more than one perforate area, viz., one chief " polar aperture," and one, two, or more accessory apertures (Fig. XVI. 5). The nucleus is single. Around the central capsule is an abundant dark brown pigment (phseodium of Haeckel). The siliceous skeleton exhibits various shapes regular and irregular, but is often remarkable for the fact that it is built up of hollow tubes. Fain. 1. PH.EOCYSTIDA, Haeck. The siliceous skeleton is either entirely absent or consists of hollow needles which are disposed outside the central capsule, regularly or irregularly. Genera (selected). Aulacantha, Haeck.; T/ialassoplancta, Haeck. Fain. 2. PH-EOGROMIDA, Haeck. The siliceous skeleton consists of a single fenestrated shell, which may be spherical, ovoid, or often dipleuric, but always has one or more large openings. Genera (selected). Cliallcngeria. "NVy. Thomson; Lithoyromia, Haeck. Fam. 3. PH^EOSPH.ffiRlDA. The siliceous skeleton consists of numerous hollow tubes which are united in a peculiar way to form a large spherical or polyhedral basket-work. Genera (selected). Aulosphsera, Haeck. (Fig. XVI. 9); Aulo- plcgrna, Haeck. ; Cannacantha, Haeck. Fam. 4. PH^EOCONCHIDA. The siliceous skeleton consists of two separate fenestrated valves, similar to a mussel s shells ; often there are attached to the valves simple or branched hollow tubes of silex. Genera (selected). Conchidium, Haeck. ; Ccelodendrum, Haeck. (Fig. XVI. 4). SUB-CLASS II. Acanthometridea, Lankester ( = Acanthino-skelcta). Characters. Radiolaria in which the skeleton is composed of a peculiar horny substance known as acanthin (rarely of silica). The central capsule is uniformly perforate (Peripylaea type). A divided or multiple nucleus is present in the capsule ; the capsule- wall is single. The skeleton always has the form of spines which radiate from a central point within the capsule where the} 1 are all fitted to one another. Rarely a fenestrated tangential skeleton is also formed. Fam. I. ACAXTHONIDA, Haeck. Skeleton consisting of twenty spines of acanthin disposed in five parallel zones of four spines each, meeting one another at the central point of the organism ; never forming a fenestrated shell. Genera (selected). Acanthometra, 3. Miiller (Fig. XVI. 6,7); Astrolonche, Haeck. ; Amphilonche, Haeck. (Fig. XIV. 18). Fam. 2. DIPLOCONIDA, Haeck. Skeleton a double cone. Genus unicum. Diploconus, Haeck. Fam. 3. DORATASPIDA, Haeck. The twenty acanthin spines of the skeleton form by transverse outgrowths a spherical fenestrated shell. Genera (selected). Stauraspis, Haeck. ; Dorataspix, Haeck. Fam. 4. SPH^EROCAPSIDA, Haeck. The twenty acanthin spines are joined together at their free apices by a simple perforate shell of acanthin. Genus unicum. Sphserocapsa. Fam. 5. LITHOLOPHIDA. Skeleton of many needles of acanthiu radiating from a single point without definite number or order. Genera. Litholophus, Haeck. ; Astrolophus, Haeck. Further remarks on the Radiolaria. It has not been possible in the systematic summary above given to enumerate the immense number of genera which have been distinguished by Haeckel (42) as the result of the study of the skeletons of this group. The important differences in the structure of the central capsule of different Radio laria were first shown by Hertwig, who also discovered that the spines of the Acanthometridea consist not of silica but of an organic com pound. In view of this latter fact and of the peculiar numerical and architectural features of the Acanthometrid skel?ton, it seems proper to separate them altogether from the other Radiolaria. The Peripylaea may be regarded as the starting point of the Radiolarian pedigree, and have given rise on the one hand to the Acantho metridea, which retain the archaic structure of the central capsule whilst developing a peculiar skeleton, and on the other hand to | the Monopylpea and Phreodaria which have modified the capsule but retained the siliceous skeleton. Pliaiodaria. Peripylttu. Acanthometridea, Arcni-peripylaea. KAOIOLAHIA. The occasional total absence of any siliceous or acanthinous skeleton does not appear to be a matter of classiticatory importance,
since skeletal elements occur in close allies of those very few forms