562 THE LOWEST ANIMALS. 



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

 three sections, according to the characters 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 provided with a flagellum or whip. Two spores, which may be of 

 similar or of different size, fuse together ; the resulting individual growing into 

 an adult radiolarian. Certain yellow corpuscles present in the outer part of the 

 body of surface radiolarians are unicellular parasitic algae, which can be separated 

 and cultivated independently 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 

 south-eastern part of the Indian Ocean the ooze forming the ocean-bed is chiefly 

 made up of their skeletons, sometimes 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. 1 of the Plate shows the elegant lattice sphere of 

 Rhizospkcera. Fig. 2 represents Sphcerozoum, whose skeleton consists of loose 

 spicules, arranged tangentially. Actinomma (Fig. 3) possesses three concentric 

 lattice-spheres, joined by radiating spikes. Fig. 7 represents a deep-sea form 

 (Challengeria), whose oval case is formed of a regular, very fine-meshed network. 

 Fig. 8 depicts the elegant lattice-sphere of Heliosphcera ; while LitJiomespilus, 

 Ommatocampe, and Carpocanium are shown in Figs 4, 5, and 6, and Clathrocydas 

 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 animal- 

 cules so named were classed with the worms and radiated animals, or zoophytes. 

 As the microscope improved, infusorians were found to possess a considerable 

 amount of structure. Ehrenberg attributed to them a highly complex organisa- 

 tion, supposing them to possess intestines, nervous system, etc. Later observations 

 negatived these views, and showed them to be animals formed of one cell or 

 composed of a colony of one-celled individuals. It is true that this cell, or unit- 

 mass of protoplasm, may show a wonderful amount of differentiation, what with its 

 nucleus, contractile vacuole, mouth and gullet, its variously arranged cilia or flagella, 

 its contractile fibres, its separation into an outer denser and an inner more fluid 

 protoplasm, and its horny cups, stalks, etc. Most of the species here described live 

 in ponds and ditches, the larger forms preferring clear to stagnant and muddy water. 



