THE INFUSORIAL ANIMALCULES 3537 



out a skeleton, and consist of small spherical or oval masses of soft gluey pro- 

 toplasm, with slender radiating pseudopods, and one or several central capsules; 

 the presence of more than one of the latter indicating a colonial form of growth. 

 In a few species the skeleton is formed of a glassy-looking horny substance, 

 termed acanthin, arranged in the form of radiating spines. 



The vast majority of species secrete a siliceous skeleton which assumes an end- 

 less variety of forms, such as trellis-work spheres, concentric spheres or boxes 

 joined by radiating spines, helmets, baskets, lanterns, beehives, discs, rings, etc. 

 Haeckel has described over four thousand species, and possibly as many more could 

 be added to this number. Radiolaria are divided into two groups; in the first of 

 these there is either no skeleton or one of silex, while in the second the skeleton 

 is formed of radiating spines of a horny nature. The first group is subdivided into 

 three sections, according to the characteristics of the central capsule. In the first 

 section the capsule is spherical and uniformly perforated by numerous small pores; 

 in the second conical, with a perforated sieve-like floor area below; and in the third 

 it has one main aperture and one or a few accessory ones, and is surrounded by a 

 dark pigment. In the forms with a siliceous skeleton the geometrical pattern of 

 the skeleton conforms more or less to the shape of the central capsule, being either 

 spherical or conical. The central capsule is regarded as being homologous with 

 the calcareous shell of Globigerina. Reproduction takes place by simple division 

 into two, or by the breaking up of the body substance into oval spores, each pro- 

 vided with a flagellum or whip. Two spores, which may be of similar or of differ- 

 ent size, fuse together; the resulting individual growing into an adult radiolarian. 

 Certain yellow corpuscles present in the outer part of the body of surface radiola- 

 rians are unicellular parasitic algae, which can be separated and cultivated independ- 

 ently of their host. The radiolarians live floating at all depths. Some forms are 

 abyssal, living in depths of one thousand to two thousand five hundred fathoms. 

 Over certain areas in the Central Pacific and the southeastern part of the Indian 

 Ocean the ooze forming the ocean bed is chiefly made up of their skeletons, some- 

 times to an extent of eighty per cent, of the deposit, which has hence been termed 

 Radiolarian ooze. The chalky-looking rock, known as Barbados earth, a Tertiary 

 formation, is composed almost entirely of skeletons of radiolarians. Somewhat 

 similar deposits exist in the Nicobar islands, in Greece, and in Sicily. Fig. i of the 

 plate shows the elegant lattice sphere of Rhizosphczra. Fig. 2 represents Spkcero- 

 zoum, whose skeleton consists of loose spicules, arranged tangentially. Adinomma 

 (Fig. 3) possesses three concentric lattice spheres, joined by radiating spikes. Fig. 

 7 represents a deep-sea form (Challengeria) t whose oval case is formed of a regular, 

 very fine-meshed network. Fig. 8 depicts the elegant lattice sphere of Heliosphcera; 

 while Lithomespilus, Ommatocampe, and Carpocanium are shown in Figs. 4, 5, and 

 6, and Clathrocyclas and Dictyophimus in Figs. 9 and 10. 



THE INFUSORIAL ANIMALCULES Class Infusoria 



The name Infusoria, which came into use a hundred years ago, was applied to 

 certain tiny living specks which appeared in infusions of hay, etc. The animalcules 



