APHROTHORACA.—_Actinolophus_ F.E. Sch.; _Myxastrum_ Haeck.; _Gymnosphaera_ Sassaki; _Dimorpha_ (Fig. 37, 5, p. 112) Gruber; _Actinomonas_ Kent; _Actinophrys_ Ehrb.; _Actinosphaerium_ St.; _Camptonema_ Schaud; _Nuclearia_ Cienk. {71}2. Invested with a gelatinous layer, sometimes traversed by a firmer elastic network. CHLAMYDOPHORA.—_Heterophrys_ Arch.; _Mastigophrys_ Frenzel; _Acanthocystis_, Carter. 3. Ectoplasm with distinct siliceous spicules. CHALAROTHORACA.—_Raphidiophrys_ Arch. 4. Skeleton a continuous, fenestrated shell, sometimes stalked. DESMOTHORACA.—_Myriophrys_ Penard; _Clathrulina_ Cienk.; _Orbulinella_ Entz. This class were at first regarded and described as fresh-water Radiolaria, but the differences were too great to escape the greatest living specialist in this latter group, Ernst Haeckel, who in 1866 created the Heliozoa for their reception. We owe our knowledge of it mainly to the labours of Cienkowsky, the late William Archer, F. E. Schulze, R. Hertwig, Lesser, and latterly to Schaudinn, who has monographed it for the "Tierreich" (1896); and Penard has published a more recent account. [Illustration: FIG. 18.—_Actinophrys sol._ _a_, Axial filament of pseudopod; _c.v_, contractile vacuole; _n_, nucleus. (From Lang's _Comparative Anatomy_, after Grenacher.)] _Actinophrys sol_ Ehrb. (Fig. 18) is a good and common type. It owes its name to its resemblance to a conventional drawing of the sun, with a spherical body and numerous close-set diverging rays. The cytoplasm shows a more coarsely vacuolated outer layer, sometimes called the ectosarc, and a denser internal layer the endosarc. In the centre of the figure is the large nucleus, to which the continuations of the rays may be seen to converge; the pseudopodia contain each a stiffish axial filament,[83] which is covered by the fine granular plasm, showing currents of the granules. The axial filament disappears when the pseudopodia are retracted or bent, and is regenerated afterwards. This bending occurs when a living prey touches and adheres to a ray, all its neighbours bending in like the tentacles of a Sundew. The prey is carried down to the surface of the ectoplasm, and {72}sinks into it with a little water, to form a nutritive vacuole. Fission is the commonest mode of reproduction, and temporary plastogamic unions are not uncommon. Arising from these true conjugations occur, two and two, as described by Schaudinn. A gelatinous cyst wall forms about the two which are scarcely more than in contact with their rays withdrawn. Then in each the nucleus divides into two, one of which passes to the surface, and is lost (as a "polar body"), while the other approaches the corresponding nucleus of the mate, and unites with it, while at the same time the cytoplasms fuse. Within the gelatinous cyst the zygote so formed divides to produce two sister resting spores, from each of which,