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in invertebrates (for example, crustaceans, arachnids, and insects), the thin layer of usually cylindrical epithelium lying immediately under the cuticle (which is formed by the secretory activity of the cells of the hypodermis). The various cutaneous glands and the tactile and olfactory hairs are located in the hypodermis. In nematodes, gastrotrichs, and several other worms, the hypodermis is a syncytium that secretes the cuticle.
In plants, the hypodermis is one or more layers of cells located under the epidermis of the stems, leaves, seeds, and fruits, or under the epiblem (piliferous layer) of the roots. The hypodermis is part of the primary cortex of stems, often consisting of cells with thickened walls, and may be classified by function as mechanical tissue. It is most characteristic of succulents. In leaves the hypodermis consists of one, or more often several, layers of cells of aquiferous tissue (for example, in a number of tropical plants) or of mechanical tissue (for example, in pines and sago palms). In the leaf the hypodermis is formed when the cells of the epidermis are divided by septa that are parallel to the surface of the leaf, such as in Ficus and Begonia, or from cells of the mesophyll of the leaf tissue lying under the epidermis, such as in some palms. In roots the outer layers of cells of the primary cortex are sometimes called the exodermis.