PROTOZOA in which the organism is provided with pseudopodia as organs of locomotion and ingestion of food, or for the latter purpose alone. Contour of the body, whether provided with an envelope or test or not, either stable or amoeboid. The pseudopodia are sometimes very stable, usually changeable, always retractile. The first stage of the life-history, or the fission-product is sometimes flagellate.

There are six classes, the Heliozoa, Radiolaria, Foraminifera, Amoe-bina, Mycetozoa, and Labyrinthulidea, and a group, Proteomyxa, including an assemblage of forms which cannot at present be located elsewhere.

Class Heliozoa

Rhizopod Protozoa with radiant and stable pseudopodia, seldom branched or anastomosing, and a globular body of unchanging outline; rarely fixed. The protoplasm is very generally divided into an ectosarc and an endosarc, not delimited by a membrane; the organism is uni- or multi-nucleate; vacuoles both contractile and non-contractile are commonly present. A skeleton ts sometimes absent; when present it is gelatinous, or siliceous, and then either composed of loose spicules, plates, etc, or of a continuous lattice-shell. An adventitious skeleton is rare. Biflagellate spores have been observed in some instances. Widely distributed; freshwater and marine.

A skeleton is absent in one section of the class, the Aphrothoraca. When present it is either gelatinous (?) or siliceous, rarely adventitious. As to the first-named, it occurs in the Chlamydophora, and takes the form of a soft more or less mobile envelope, homogeneous, granular or striated; said to be separated from the body by a space (?a homogeneous jelly). There is some doubt as to its constitution. Three facts appear to point to its organic nature: (1) the presence in Heterophrys of minute points or processes of its free surface which contract under the action of carmine staining fluid; (2) its constant elevation round the bases of the pseudo-podia; (3) its occurrence in a loose and crumpled condition round an encysted Sphaerastrum. On the contrary it has been said to consist in Heterophrys of a coat of finely interlaced needles which resist the action of strong sulphuric acid, and are consequently supposed to be siliceous. An undoubtedly siliceous skeleton exists in the two remaining sections of the class, the Chalarothoraca and Desmothoraca. In the former, it is composed of loose parts: spherules in one to three layers (Pompholyxophrys pumicea - Hyalolampe); round plates in a single layer (Pinacocystis rubi-cundd); leaf-like plates pointed at each end and pierced by pores (?) (Pinaciophora fluviatilis); spicules placed tangentially to the surface, straight or curved, and pointed at both ends (Raphidiophrys); spines radially placed, with basal plates, and either simple or forked at their outer extremities, together with tangential spicules (Acanthoeystis). These loose structures are held together either by protoplasm derived from the pseudopodia, or by a soft secreted material; the fact that the parts retain their position when the animal is contracted and encysted favours the latter view.

In Raphidiophrys the skeleton is so loose that it is raised into eminences where the pseudopodia protrude. A clear interval may separate the skeleton from the bocFy of the animal, whether a space or homogeneous jelly is doubtful. In the Desmothoraca the skeleton is a lattice-sphere through the apertures of which pass the pseudopodia. It turns brown with age and is attached by a siliceous peduncle in Clathrulina; it is free in Orbulinella; and in both instances it is larger than the body of the animal. An adventitious skeleton of sand-grains occurs in the marine Lithocolla and Raphidiophrys arenosa, of sand-grains mixed with diatom-shells in the freshwater Elaeorhanis.

The animal is attached by a chitinoid peduncle in the skeleton-less Actinolophus, otherwise it is free though inclosed in a fixed shell in Clathrulina. The protoplasm, which varies in consistence, and is rarely coloured of a yellow tint, is clearly divisible into an ectosarc, and an endosarc in Actinophrys, Actinosphaerium and Actinolophus, and probably in all skeletogenous forms except the Desmothoraca. The ectosarc is more or less granular, and the seat of digestion, the endosarc, homogeneous; in Actinosphaerium, however, the characters of the two parts are reversed. In Actinolophus and Acanthoeystis the endosarc is excentric and touches the surface at one spot, in others it is central. There is no separating membrane between the two parts, yet the transition from one to the other is usually somewhat sudden. Some species are coloured: Actinolophus often contains numerous orange particles in its ectosarc; Chondropus peculiar green spherules; Acanthocystis flava reddish-yellow granules; Pinacocystis, etc. reddish-brown; Pomp holyxopkrys exigua a single central ruby-red globule.

Chlorophyl bodies occur occasionally in Actinophrys and sometimes in the endosarc of Actinosphaerium, constantly in the ectosarc of Acanthocystis and Heterophrys myriapoda, in Sphaeratrum Fockii, Raphidiophys viridis, and R. elegans. But colourless forms of these species occur, and in this case colourless bodies, resembling the chlorophyl bodies may be present 1. Contractile vacuoles have not been observed in Actinolophus, nor in all the species of some skeletogenous genera, e. g. of Raphidiophrys. As to their number, Actinophrys has one, Actinosphaerium one to five; but larger numbers are sometimes found, e. g. twenty in Raphidiophrys pallida. They are either deeply placed in the ectosarc, are not large, and do not bulge the surface, or they are quite superficial, swell to a very great size, e. g. in Actinophrys the vacuole may almost equal the body in diameter and project freely above the surface. The new vacuole appears in the same place as the old, and may originate from a remnant of it. Non-contractile vacuoles are sometimes absent, sometimes sparingly present, in Actinolophus and most skeletogenous genera.