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The cell itself consists of two halves, each containing an essentially flat plate, or valve, and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical.
Cells are rectangular in girdle view (when in colonies), and lanceolate in valve view. Raphe system is slightly keeled and runs from pole to pole. Two large plate-like chloroplasts are present, one near each end of the cell. The nucleus is located centrally. Cells are yellow-brown in colour.
A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica , made from silicic acid , and is coated with a layer of organic substance, which was referred to in the early literature on diatoms as pectin , a fiber most commonly found in cell walls of plants .
The microscopic layers of that shield appear to evolutionary biologists, "like they are composed of little tooth-like structures." [4] Neil Shubin writes: "Cut the bone of the [ostracoderm] skull open…pop it under a microscope and…you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp.
For many years the diatoms—treated either as a class (Bacillariophyceae) or a phylum (Bacillariophyta)—were divided into just 2 orders, corresponding to the centric and the pennate diatoms (Centrales and Pennales; alternative names Biddulphiales and Bacillariales, as used e.g. in Lee, 1989). [9]
Navicula diatoms are highly motile and move through a gliding movement [3] [4] [5] This is done through excretion of extracellular polymeric substances (EPS). One form of EPS surrounds the outside of the cell and another is excreted through a slit in the frustule called a raphe, allowing the cell to glide along a track.
Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in a respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different.
Going one step even further back, the chromerids, the peridinin dinoflagellates and the heterokont algae have been argued to possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a red alga, [20] and so it seems likely that the common ancestor of alveolates and heterokonts was also photosynthetic.