A class of the phylum Radiozoa in the Actinopoda. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the Actinopoda (originally a class) containing two phyla: Heliozoa and Radiozoa. Within the Radiozoa is the class Acantharea. These marine protozoans possess a nonliving, organic capsular wall surrounding a central mass of cytoplasm. The intracapsular cytoplasm is connected to the extracellular cytoplasm by fine cytoplasmic strands passing through pores in the capsular wall. When viewed with the electron microscope, the capsular wall in some species appears to be made of many layers, each composed of a fine fibrillar network. The skeletons are made of celestite (strontium sulfate) instead of silica. The basic structural elements are 20 rods that pass through the capsule to the center in regular arrangements (polar and equatorial; see illustration). An equatorial rod forms a 90° angle with a polar rod, and other groups are arranged with similar exactness. This type of cytoskeleton may be modified by the addition of a latticework, composed of plates, each fused with a skeletal rod. Some genera show a double latticework, concentric with the central capsule. The skeletons do not contribute substantially to the paleontological record in marine sediments since celestite is dissolved by seawater. Dissolution is pronounced below 617 ft (188 m) depth. While the protozoan is still alive, however, the cytoplasm appears to protect the skeleton from dissolution.

A taxon of marine protozoans belonging to the Acantharea. Acanthometrida (also known as Arthracanthida) is a taxonomic grouping (sometimes considered as an order or subclass) placed within the Acantharea (Acantharia) taxon. All members are unicellular planktonic marine protozoa with axopodia (rigid, linear cellular projections). Long slender pseudopodia (temporary projections) are found in certain members as well. Mitochondria are always present and contain flattened cristae. No cilia are present in the trophic phase. Members have a skeleton, made up of strontium sulfate (celestite), that is limited to 20 radially arranged rods that extend from the center, forming a characteristic pattern in which angles are quite exact, as in Acanthometra (see illustration). Because strontium sulfate is soluble in seawater, no acantharean fossils exist. The central endoplasm contains numerous nuclei and other organelles. The cytoplasm surrounding the spines contains a conical array of contractile microfilaments (myophrisks or myonemes that are calcium-activated) containing the contractile protein actin. The microfilaments expand or contract the gelatinous sheath surrounding the cell, which allows for changes in the level of flotation. Electron microscopic research shows that the microfilaments are arranged in two sets of bands; one set runs parallel to the axis of the spine, and the other runs crosswise, thus producing a cross-fibrillar network. Cysts and flagellated swarmers represent two stages of the life cycle, which is not completely known at present. The endoplasm often contains zooxanthellae (small dinoflagellates living in the protozoan's endoplasm). See also: Acantharea; Celestite; Protozoa; Strontium

An order of Acantharia. In this group of protozoa, skeletons typically include a latticework shell, although the characteristic skeletal rods are recognizable. The latticework may be spherical or ovoid, is fused with the skeletal rods, and is typically concentric with the central capsule. The body is usually covered with a single or double gelatinous sheath through which the skeletal rods emerge. Myonemes (usually a specific number) extend from the gelatinous sheath to each skeletal rod. These marine forms live mostly below depths of 150–200 ft (45–60 m). The order includes Coleaspis, Diploconus, Dorotaspis, and many other genera. See also: Acantharea; Actinopodea; Protozoa; Sarcodina; Sarcomastigophora

A major constituent of the cell walls of certain red algae, especially members of the families Gelidiaceae and Gracilariaceae. Extracted for its gelling properties, it is one of three algal polysaccharides of major economic importance, the others being alginate and carrageenan.

An informal assemblage of predominantly aquatic eukaryotes that carry out oxygen-evolving photosynthesis, but lack specialized water-conducting and food-conducting tissues. Algae (Fig. 1) are eukaryotic organisms (having an organized nucleus). Previously, taxonomists thought that algae were plants. However, algae are now considered to be plantlike protists (eukaryotes that are not fungi, animals, or plants). With regard to general form and structure, algae range from unicells that are 1–2 μm in diameter to huge thalli [vegetative bodies; for example, kelps measuring 30 m (100 ft) in length] with functionally and structurally distinctive tissues and organs. Unicells may be solitary or colonial, attached or free-living, with or without a protective cover, and motile or nonmotile. Colonies may be irregular or they may display a distinctive pattern, with the latter type being flagellate or nonmotile. Multicellular algae form packets, branched or unbranched filaments, sheets that are one or two cells thick, or complex thalli, including some with organs resembling roots, stems, and leaves (as in the brown algal orders Fucales and Laminariales). Coenocytic algae, in which the protoplast is not divided into cells, range from microscopic spheres to thalli that are 10 m (33 ft) in length with a complex structure of intertwined siphons (as in the green algal order Bryopsidales). Algae differ from the next most advanced group of organisms, the bryophytes (nonvascular land plants), by their lack of multicellular sex organs sheathed with sterile cells and by their failure to retain an embryo within the female organ. Many colorless organisms are referable to the algae on the basis of their similarity to photosynthetic forms with respect to structure, life history, cell wall composition, and storage products. The study of algae is called algology (from the Latin alga, meaning sea wrack) or phycology (from the Greek phykos, meaning seaweed). See also: Bryophyta; Bryopsidales; Cell nucleus; Eukaryota; Fucales; Laminariales; Photosynthesis; Protist

A major constituent (10–47% dry weight) of the cell walls of brown algae. Extracted for its suspending, emulsifying, and gelling properties, it is one of three algal polysaccharides of major economic importance, the others being agar and carrageenan. See also: Agar; Carrageenan

Any protozoan moving by means of protoplasmic flow. In their entirety, ameboid protozoans (or ameboid protists) include naked amebas, those enclosed within a shell or test, and more highly developed representatives, such as heliozoians, radiolarians, and foraminiferans. Ameboid movement is accomplished by pseudopods—cellular extensions that channel the flow of protoplasm. Pseudopods take varied forms and help distinguish among the different groups. A lobe-shaped extension or lobopod is perhaps the simplest type of pseudopod (see illustration). The shapelessness and plasticity of these locomotory organelles impart an asymmetric, continually changing aspect to the organism. Other, more developed, representatives have pseudopodial extensions containing fibrous supporting elements (axopods) or forming an extensive network of anastomosing channels (reticulopods). Although involved in locomotion, these organelles are also functional in phagocytosis—the trapping and ingesting of food organisms (usually bacteria, algae, or other protozoa) or detritus. See also: Amoebida; Foraminiferida; Heliozoia; Phagocytosis; Protist; Protozoa; Radiolaria

An order of Lobosia without protective coverings (tests). These protozoa range in size from about 4 micrometers to 0.08–0.12 in. (2–3 mm). Pellicles may be thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba verrucosa (Fig. 1a). Pellicular folds may develop during locomotion, particularly in species with thick pellicles. Both flagellate and ameboid stages occur in certain soil amebas. In Naegleria, for example, the cycle includes an ameba, a flagellate, and a cyst. Most Amoebida have no flagellate stage.

An order of Granuloreticulosia in which the naked amebas form branched, threadlike, interconnected pseudopodia (reticulopodia). Species are known from freshwater, salt water, and brackish water. General characteristics of the order Athalamida are difficult to select, and the order may lack the taxonomic stability to survive accumulation of more adequate information. Heterogeneity may extend even to a genus; described species of Biomyxa differ appreciably in morphology and exhibit, for example, uninucleate and multinucleate conditions in different species (although these may represent young and mature stages in life cycles). The granular cytoplasm in Biomyxa (see illus.a) shows no clear separation into ectoplasm and endoplasm. In addition to Biomyxa, the genera Arachnula (see illus.b), Gymnophrys, and Pontomyxa have been assigned to this order. See also: Ameba; Protozoa; Rhizopodea; Sarcodina

A class of nonflagellate unicellular algae, commonly called diatoms, with boxlike silicified walls. Members of the algal class Bacillariophyceae are commonly termed diatoms (Fig. 1). Diatoms range in maximum dimension from 4 μm (0.00004 in.) to more than 1 mm (0.04 in.). The diatom wall or frustule comprises several interlocking, usually elaborately sculptured, lightly or heavily silicified pieces overlying a thin polysaccharide layer. The two largest pieces are the upper and lower valves, which fit together like the top and bottom of a petri dish or shoe box. Between the valves (along the side or girdle of the cell), several smaller pieces—hooplike girdle bands—are intercalated. Depending upon which dimension is larger, that is, breadth or depth, a diatom tends to lie on the valve side or on the girdle side. See also: Algae; Polysaccharide