{54}[Illustration: FIG. 9.—_Trichosphaerium sieboldii._ 1, Adult of "A" form; 2, its multiplication by fission and gemmation; 3, resolution into 1-nucleate amoeboid zoospores; 4, development (from zoospores of "A") into "B" form (5); 6, its multiplication by fission and gemmation; 7, its resolution after nuclear bipartition into minute 2-flagellate zoospores or (exogametes); 8, liberation of gametes; 9, 10, more highly magnified pairing of gametes of different origin; 11, 12, zygote developing into "A" form. (After Schaudinn.)] Several genera have tests of siliceous or chitinous plates, formed in the cytoplasm in the neighbourhood of the nucleus, and connected by chitinous cement. Among these _Quadrula_ (Fig. 10, A) is Lobose, with square plates, _Euglypha_ (Fig. 8, p. 29), and _Paulinella_[69] are Filose, with hexagonal plates. In the latter they are in five longitudinal rows, with a pentagonal oral plate, perforated by the oval pylome. In other genera again, such as _Cyphoderia_ (Filosa), the plates are merely {55}chitinous. Again, the shell may be encrusted with sand-grains derived directly from without, or from ingested particles, as shown in _Centropyxis_, _Difflugia_ (Fig. 10, D), _Heleopera_, and _Campascus_ when supplied with powdered glass instead of sand. The cement in _Difflugia_ is a sort of organic mortar, infiltrated with ferric oxide (more probably ferric hydrate). In _Lecqueureusia spiralis_ (formerly united with _Difflugia_) the test is formed of minute sausage-shaped granules, in which may be identified the partly dissolved valves of Diatoms taken as food; it is spirally twisted at the apex, as if it had enlarged after its first formation, a very rare occurrence in this group. The most frequent mode of fission in the testaceous Rhizopods (Figs. 8, 10) is what Schaudinn aptly terms "bud-fission," where half the protoplasm protrudes and accumulates at the mouth of the shell, and remains till a test has formed for it, while the other half retains the test of the original animal. The materials for the shell, whether sand-granules or plates, pass from the depths of the original shell outwards into the naked cell, and through its cytoplasm to the surface, where they become connected by cementing matter into a continuous test. The nucleus now divides into two, one of which passes into the external animal; after this the two daughter-cells separate, the one with the old shell, the other, larger, with the new one. [Illustration: FIG. 10.—Test-bearing Rhizopods. A, _Quadrula symmetrica_: B, _Hyalosphenia lata_; C, _Arcella vulgaris_; D, _Difflugia pyriformis_. (From Lang's _Comparative Anatomy_.)] If two individuals of the shelled species undergo bud-fission in close proximity, the offspring may partially coalesce, so that a monstrous shell is produced having two pylomes.