all 282 different fossil forms have been described, grouped in 44 genera. _Fig. 92, p. 21._ [Illustration: AULOCANTHA SCOLYMANTHA.] _Fig. 93, p. 21._ [Illustration: ACTINOMMA DRYMODES.] _Fig. 94, p. 21._ [Illustration: HALIOMMA ECHINASTER.] In certain Polycystina, the perforations of the shell are so large and so close together, that the sarcode body of the animal appears to be covered by a siliceous net. This connects them with the Thalassicollæ, minute creatures found passively floating on the surface of the sea. Th. morum, which is one of the most simple of the few forms known, has a spherical body of sarcode covered with a siliceous net, through which the pseudopodia radiate in all directions, as in the Actinophrys, but it is studded at regular distances with groups of apparently radiating siliceous spicules. The Aulocantha scolymantha (fig. 92), found by M. Haeckel in the Mediterranean, may be taken as an example of the most general form of the Thalassicolla. The siliceous skeleton of some of the Radiolaria resembles the Chinese ivory toy of ball within ball. That of the Actinomma drymodes (fig. 93) consists of three perforated concentric spheres, with six strong spicules attached to the outer surface, perpendicular to one another and prolonged in the interior to the central sphere. Hundreds of finer bristle-like spicules radiate from the surface. The animal is chiefly contained in the central sphere, and from it a perfect forest of fine, long pseudopodia radiate in thick tufts through the apertures of the exterior sphere. The skeleton of the Haliomma (fig. 94) consists of only two concentric spheres. In many species of Haliomma and Actinomma the animals are of the most vivid vermilion or purple colour. Little or nothing is known of the reproduction of these microscopic organisms. The Actinomma drymodes and the Haliomma are two of the most beautiful microscopic rhizopods discovered by M. Haeckel. There is a family of fresh-water testaceous rhizopods of which one group secretes its shell and the other builds it. The horny shell secreted by the group of the Arcella presents various degrees of plano-convexity, the convexity in some cases amounting to a hemisphere. They rarely, if ever, have mineral matter on their surface, which is studded with regular but very minute hexagonal reticulations. The aperture or mouth of the shell is small, and invariably occupies the centre of the plane surface, its margins being more or less inverted. The form of the shell is exceedingly varied, sometimes it even has horns indefinite in number, sometimes symmetrical, sometimes not; when its test or covering becomes too small for its increasing size, it quits it, and secretes a new one. The filamental pseudopodia proceed from the mouth of the shell only, and by means of these it creeps about on its mouth in search of food. [Illustration: Fig. 95. Simple Rhizopods.—A, B, Difflugiæ; C, D, Arcellæ.]