The Heliozoa may be defined as Rhizopoda, which possess a contractile vesicle, and are devoid of a central capsule. The body is naked, or is provided with skeletal structures of a variable nature, but sometimes siliceous. The pseudopodia stand out like rays, but may anastomose with one another.
In their radiant pseudopodia and in the occasional presence of siliceous spicules, the Heliozoa are allied to the typical Radiolarians; but the absence of a central capsule and the presence of a contractile vesicle approximate them to the Amaebea; while the absence of "yellow cells," as also of a gelatinous outer investment to the sarcode, distinguish them further from the true Radiolarians. They must therefore be regarded as an inosculating group, related on the one hand to the Amaebea, and on the other to the Radiolaria.
Most of the Heliozoa are inhabitants of fresh water, and we may select as a type the common "Sun-animalcule" (Actin-ophrys sol), in which no hard structures are developed. In this animalcule (fig. 21), the body consists of a spherical mass of sarcode, about 1-1300 of an inch in diameter, and usually covered with long, radiating, filamentous pseudopodia, which are much less mobile than in the case of the Amoeba. The division of the substance of the body into ectosarc and endo-sarc is tolerably evident, and the latter contains numerous granules and vacuoles. The pseudopodia are derived from the ectosarc alone, the endosarc not passing into them, and they exhibit a circulation of granules along their edges, though this is not nearly so marked a feature as in the case of the Foraminifera. A nucleus and contractile vesicle are also present.
Fig. 21. - Actinophrys sol, showing the radiating pseudopodia. One specimen has swallowed a Diatom.
Actinophrys occurs in both fresh and salt water. Actitio-sphaerium is in many respects like Actinophrys, but each of the pseudopodia is supported upon a strong albuminous spine; and the sarcode of the body is vesicular or "alveolar," while numerous nuclei exist in the central sarcode. In Heterophrys (fig. 22), there is a globular body, the ectosarc of which is surrounded by a kind of external investment or excretion, which appears to be of a protoplasmic nature, but takes no part in the production of the pseudopodia. The latter are long, granular, and unbranched, and amongst them are long spine-like processes of firm sarcode, which have been regarded as of a chit-inous nature. Acanthocystis is, like the preceding, a fresh-water form, but it possesses long radiating siliceous spines; while Clathrulina has the body enclosed in a regular fenestrated siliceous test, which is supported upon a siliceous peduncle.
Fig. 22. - Heterophrys spinifera, one of the Heliozoa, greatly enlarged. (After Hertwig and Lesser.) c c Contractile vesicles.