Invertebrates

Characteristics of diatoms:

• All species are unicellular or colonial coccoid algae. None are free-living flagellates.
• The only flagellate cells produced are the male gametes (= sperm, spermatozoids) of 'centric' diatoms. These have a single forward-pointing flagellum, which bears mastigonemes.
• The relative proportions of the chlorophylls and fucoxanthin produce a yellow-brown or greenish-brown colour in the plastids.
• Most have a large central vacuole or pair of vacuoles.
• Cells (especially during stationary-phase) often accumulate large quantities of lipids and fatty acids; polyphosphate bodies are also present and sometimes take the form of discrete spherical or complex 'volutin' granules, one per vacuole.
• Secretion of extracellular polymeric material (usually polysaccharides) is common, as stalks, pads, capsules, tubes, chitin fibres, or trail material from locomotion.
• All cells (except the gametes and endosymbiotic diatoms) possess a bipartite cell wall comprising two overlapping halves.
• Each half-wall itself consists of a large end-piece, the 'valve', and several or many narrow bands or segments, which together form the 'girdle'.
• The cell wall is almost always heavily silicified.
• Cell wall elements (valves, girdle bands, and auxospore scales and bands) are formed intracellularly, in special membrane-bound 'silica deposition vesicles' associated very closely with the cell membrane; they are not secreted from the cell until they are complete.
• New wall elements are always produced within the confines of an existing cell wall. As a result, average cell size usually decreases with successive mitotic divisions during the life cycle.
• Size is restored via the formation and expansion of a special cell, the auxospore, which is usually a zygote. The basic shape of each diatom species is largely created during the expansion of the auxospore, but is often modified during subsequent mitotic cell divisions.
• During vegetative mitoses, the nucleus always lies to one side of the cell immediately beneath the girdle, at the edge of the hypotheca.
• Mitosis is open, the nuclear envelope breaking down before metaphase; the spindle is a narrow cylinder, persistent at telophase, consisting of two interdigitating half-spindles, each associated with a polar plate.
• The chromosomes bunch closely around the cylindrical spindle at metaphase, becoming impossible to separate and count.
• Cytokinesis occurs through cleavage.
• The life cycle is strictly diplontic: as far as is known, all vegetative cells of all species are diploid, and all mitoses take place in the diploid phase. However, haploids have occasionally been grown in culture in a few species.
• They occur just about everywhere in aquatic and damp terrestrial habitats, providing that photosynthesis is possible.
• They are amazingly diverse, with hundreds of genera and perhaps 200,000 species.

(from tolweb.org)