Fig. 19.—Actinosphaerium eichornii. A, entire animal with two contractile vacuoles (c.vac); B, a portion much magnified, showing alveolate cytoplasm, pseudopodia with axial rods, non-nucleate cortex (cort), multiple nuclei (nu) of endoplasm (med), and food-vacuole (chr). (From Parker and Haswell.)
The axial rods of the pseudopodia may pass either to the circumference of the nucleus or to a central granule, corresponding, it would appear, to a centrosome or blepharoplast; or again, in the plurinucleate marine genus Camptonema, each rod abuts on a separate cap on the outer side of each nucleus. The nucleus is single in all but the genera Actinosphaerium, Myxastrum, Camptonema, and Gymnosphaera. The movements of this group are very slow, and are not well understood. A slow rolling over on the points of the rays has been noted, and in Camptonema they move very decidedly to effect locomotion, the whole body also moving Amoeba-fashion; but of the distinct movements of the species when floating no explanation can be given. The richly vacuolate ectoplasm undoubtedly helps to sustain the cell, and the extended rays must subserve the same purpose by so widely extending the surface. Dimorpha (Fig. 37, 5, p. [112]) has the power of swimming by protruding a pair of long flagella from the neighbourhood of the eccentric nucleus; and Myriophrys has an investment of long flagelliform cilia. Actinomonas has a stalk and a single flagellum in addition to the pseudopodia; these genera form a transition to the Flagellata.
Several species habitually contain green bodies, which multiply by bipartition, and are probably Zoochlorellae, Chlamydomonadidae of the same nature as we shall find in certain Ciliata (pp. [154], [158]) in fresh-water Sponges (see p. [175]), in Hydra viridis (p. [256]), and the marine Turbellarian Convoluta (Vol. II. p. 43).
Reproduction by fission is not rare, and in some cases (Acanthocystis) the cell becomes multinuclear, and buds off 1-nucleate cells. In such cases the buds at first lack a centrosome, and a new one is formed first in the nucleus, and passes out into the cytoplasm. These buds become 2-flagellate before settling down. In Clathrulina the formation of 2-flagellate zoospores has long been known (Fig. 20, 3). In Actinosphaerium (Figs. 19, 21), a large species, differing from Actinophrys only in the presence of numerous nuclei in its endoplasm, a peculiar process, which we have characterised as endogamy, results in the formation of resting spores. The animal retracts its rays and encysts; and the number of nuclei is much reduced by their mutual fusion, or by the solution of many of them, or by a combination of the two processes. The body then breaks up into cells with a single nucleus, and each of these surrounds itself with a wall to form a cyst of the second order.
Fig. 20.—Various forms of Heliozoa. In 3, a is the entire animal and b the flagellula; c.vac, contractile vacuole; g, gelatinous investment; nu, nucleus; psd, pseudopodia; sk, siliceous skeleton; sp, spicules. (From Parker and Haswell, after other authors.)
Each of these divides, and the two sister cells then conjugate after the same fashion as in Actinophrys, but the nuclear divisions to form the coupling nucleus are two in number, i.e. the nucleus divides into two, one of which goes to the surface as the first polar body, and the sister of this again divides to form a second polar body (which also passes to the surface) and a pairing nucleus.[[84]] The two cells then fuse completely, and surround themselves with a second gelatinous cyst wall, separated from the outer one by a layer of siliceous spicules. The nucleus appears to divide at least twice before the young creep out, to divide immediately into as many Actinophrys-like cells as there were nuclei; then each of these multiplies its nuclei, to become apocytial like the adult form.
Fig. 21.—Diagram illustrating the conjugation of Actinosphaerium. 1, Original cell; 2, nucleus divides to form two, N2N2; 3, each nucleus again divides to form two, N3 and n3, the latter passing out with a little cytoplasm as an abortive cell; 4, repetition of the same process as in 3; 5, the two nuclei N4 have fused in syngamy to form the zygote nucleus Nz.
Schaudinn admits 24 genera (and 7 doubtful) and 41 species (and 18 doubtful). None are known fossil. Their geographical distribution is cosmopolitan, as is the case with most of the minute fresh-water Protista; 8 genera are exclusively marine, and Orbulinella has only been found in a salt-pond; Actinophrys sol is both fresh-water and marine, and Actinolophus has 1 species fresh-water, the other marine. One of the 14 species of Acanthocystis is marine; the remaining genera and species are all inhabitants of fresh water.[[85]]