874 THE ANIMAL KINGDOM. 



p. 73; cf. note 2, p. 870 ante\ of Clathrulina, Foulke, A. N. H. (5), xiv. 1884. 

 Budding in Acanthocystis, R. Hertwig, J. Z. xi. 1877, pp. 337-40; Korotneff, 

 A. Z. Expt. viii. 1879-80, p. 481. Spores and amoebiform young, see note 2, p. 870 

 ante. 



CLASS RADIOLARIA. 



Rhizopod Protozoa, with the body divided into a central part or capsule 

 lodging the nucleus, and an extra-capsular region, by a membranous capsule 

 only exceptionally absent. Capsular -membrane pierced by fine pores, or by 

 one or several apertures. Extra-capsular region supported by a gelatinous 

 skeleton or calymna. Pseudopodia for the most part radiant, sometimes 

 branching and anastomosing. Skeleton seldom absent, either spicular or con- 

 tinuous, composed of acanthin or of silica. Reproduction by flagellate uni- 

 nucleate spores produced in the central capsule. Solitary or colonial ; ex- 

 clusively marine. 



Skeletal elements are rarely absent, as in Colloidea, Nassoidea, and 

 Phaeodinida. They consist in Acantharia of an organic substance, acanthin, 

 destroyed by a red heat and by acids ; it is supposed to be either identical 

 with vitellin (Brandt) or akin to chitin (Haeckel). This Acantharian 

 skeleton takes the form of solid radial spines, which meet in the centre 

 of the central capsule and are connected at their bases in one of four 

 ways (i) by simple apposition, (2) combined also with leaf-like ad-central 

 processes, (3) by fusion of all, or (4) of opposite pairs, the last a mode seldom 

 found. In number the spines in question are usually twenty, disposed 

 in a regular manner according to Miiller's law 1 ; indefinite in number and 

 arrangement only in the Acanthometrid family Actinelida. They are often 

 provided with lateral outgrowths (apophyses), which may be incomplete 

 or united in the Acanthophracta to form a lattice-shell. But in the family 

 Sphaerocapsida the shell is composed of small plates, each pierced by a 

 pore and united by a cement ; the spines undergo partial atrophy in two 

 genera, complete atrophy in a third, twenty apertures, however, marking 

 the places where they should protrude. In all other Radiolaria the skeleton 

 consists of silica, which is probably deposited in an organic matrix 2 ; that 

 of Phaeodaria is blackened by heat, stained by carmine, and destroyed by 



1 This law may be thus expressed. Imagine a globe with an axis of rotation, and five circles 

 inscribed on it, an equatorial, two tropical and two polar. The twenty spines lie four in each of 

 these circles, the equatorial and polar spines in the same meridian lines, 90 apart, i. e. at o, 90, 

 180, 270; the tropical in meridian lines exactly intermediate, i.e. at 45, 135, 225, 315. The 

 twenty spines are rarely all equal. See Haeckel, 'Challenger Reports,' xviii. pp. 717-20, for a 

 summary of the variations known. 



2 For the organic matrix, see Brandt, ' Kolonie-bild. Radiolarien,' Fauna und Flora des Golfes 

 von Neapel, xiii. p 63. Calcareous bodies, shaped like the rowel of a spur, occur in the calymna 

 of some Radiolaria. They are supposed by Haeckel, who terms them ' calcastrellae,' not to be 

 skeletal elements, but either unicellular algae, or of foreign origin. See HaeckeJ, op. cit. supra, 

 p. Ixx, note D. 



