There are many in Actinophrys and Actinosphaerium, confined to the ectosarc in the former, in the latter small and irregular in the endosarc, large, radially arranged, and close set in the ectosarc. In both forms the largest are peripheral. A young Actinosphaerium, however, has but a single radial layer. The ectosarcal vacuoles disappear under mechanical and electrical stimuli. A nucleus has not been observed in some of the little known forms. It is single in most instances, but specimens occur with two or sometimes with several nuclei, e. g. of Raphidiophrys pallida. Acanthocystis Italica has been only seen with several nuclei; Actinosphaerium, except in the youngest stage, is always multinucleate; a large example of it may contain 1-200 nuclei, and one, 0.85 mm. in size, is said by Carter to have had between 3-400. They multiply by binary fission and with mitosis. The nucleus always lies in the endosarc, centrally or excentrically as in Actinolophus, Acanthocystis, and Raphidiophrys. It is vesicular and has in the resting phase a distinct nuclear membrane, a nucleolus and clear nuclear fluid.
1 Are these bodies a part of the Heliozoon, or are they symbiotic Algae ? They multiply by fission, but so do chlorophyl bodies (Greeff, A.M. A. xi. p. 12); they are very generally accompanied by colourless bodies resembling them; and when they are absent, similar colourless bodies are as a rule, if not always, present. The occasional expulsion of some of them in Acanthocystis viridis has been witnessed by Greeff (A. M. A. v. p. 484); so, too, of certain yellow bodies in A. spinifera (Id. op. cit. p. 494). Entz states that A. turfacea (viridis) never ingests food (Biol. Centralblatt. ii. p. 463); that he has seen the chlorophyl bodies expelled from A.aculeata before encystation, their subsequent multiplication, and rupture of the spinose skeleton (op. cit. p. 459). On the other hand, Greeff has seen them in an encysted A. viridis (A. M. A. v. p. 489). Hertwig and Lesser observed a colourless Acanthocystis turn green with chlorophyl bodies derived from digested algal spores (A. M. A. x. suppl. p. 203). Leidy has figured a specimen containing bright red bodies, with a few green (Freshwater Rhizopoda of N. America, PI. xliii. fig. 9). At present the question cannot be regarded as decided.
The pseudopodia are fine, straight, radiant, and developed from the whole surface except in Actinolophus near the peduncle. They rarely branch or anastomose. Their length varies from about one half the diameter of the body, e.g. in Actinosphaerium to two or three times its diameter, e. g. in Acanthocystis and Clathmlina. They can be displaced from their radial position by currents of water, or bent at a sharp angle by the sudden impact of a foreign body. They are supported by axial filaments, which extend close to or up to the nucleus in Actinophrys; to just within the limits of the endosarc in Actinosphaerium, or to a central corpuscle which unites them all, in Actinolophus, Acanthocystis, Raphidiophrys, and perhaps Clathmlinal. In most cases they show a more or less lively granule-stream. The movements of the Heliozoa, whether on a solid surface or whilst floating suspended in water, are as a rule extremely slow and as yet by no means explained. Some Heliozoa appear to feed preferentially on animals, others on plants (algae, &c), others indifferently upon either. The pseudopodia draw the prey towards the body, and in Actinosphaerium into a pit which closes over it.
In Actinophrys a special broad pseudopodium-like process incloses the food, and the same method is said to obtain sometimes in Actinosphae ium, Acanthocystis and Raphidiophrys. There is generally a food vacuole. Faecal residues are expelled anywhere, and have been seen extruded along a pseudo-podium in Acanthocystis aculeata (Hertwig and Lesser).
The species of Raphidiophrys and Sphaerastrum frequently occur in a colonial condition. The skeletal envelopes of the individuals in a given colony are fused. Each individual is united to its neighbours by slender protoplasmic cords, and the pseudopodia radiating from the colony come only from the outer surfaces of the individuals. Two or more individuals of Actinophrys may become united by broad protoplasmic bridges in which both vacuoles and food may be seen. These colonies may change their shape, divide into sub-colonies, or their constituent individuals separate. Partial or complete fusion between individuals of Actinosphaerium has been observed, as well as between Actinosphaeria produced by the fission of individuals artificially brought together.
1 The axial filaments have been carefully investigated in Actinosphaerium by Brandt. They are organic, not mineral, in composition. A new pseudopodium commences as a conical process; a needle-like structure within it is the first trace of its filament. A newly formed filament is readily dissolved again by the protoplasm, an old one is more resistent: the former consists of vitellin (?), the latter contains an admixture of other bodies. New filaments may fuse inter se, they may shorten by contraction, and then either thicken, or display thickened nodes. The filaments, whether old or young, are completely absorbed when the pseudopodia are retracted in encystation. It is possible that they may also be partially dissolved during an accidental contraction of the pseudopodia.
Separation of the fused individuals usually ensues in a few hours. It is doubtful whether the phenomena detailed have any connection with reproduction. Encystation with its consequences (infra) may ensue, however, in Actinosphaeria made to fuse artificially 1.
Binary fission has been observed in comparatively few Heliozoa, Actinophrys, Actinosphaerium, Acauthocystis, Clathridina, and Hedriocystis. If Actinosphaerium is disturbed during the process, the incompletely separated individuals reunite. In Hedriocystis transverse fission has been seen in a young and still shell-less individual. The two (or four) parts into which the individual divides in Clathridina, retract their pseudo-podia if they are not retracted before fission occurs, and one or all then quit the shell, become Actinophrys-like, and finally develope a new peduncle and shell. The young Clathridina frequently attaches itself to the old shell. Fission in the same animal may also give rise to three or four parts; if to three the larger part remains in the shell, the two smaller quit it, as do all the parts when four in number. But the escaped portions become biflagellate (uniflagellate, Foulke) spores, with a single nucleus and some posteriorly placed contractile vacuoles; they eventually attach themselves and develope pseudopodia, a peduncle, and shell. Actinosphaerium may be multiplied by artificial fission.