The word fungus usually invokes images of mushrooms and toadstools or some sort of spongy, creeping growth. Although mushrooms are fungi, the forms which a fungus may take are varied. There are over 100,000 species of described fungi and probably over 200,000 undescribed. Most fungi are terrestrial, but they can be found in every habitat worldwide, including marine (500 spp.) and freshwater environments. Fungi are nonmotile, filamentous eukaryotes which lack plastids and photosynthetic pigments. The majority of fungi are saprophytes; they obtain nutrients from dead organic matter. Other fungi survive as parasitic decomposers, absorbing their food, in solution, through their cell walls. Most fungi live on the substrate upon which they feed. Numerous hyphae penetrate the wood, cheese, soil, or flesh in which they are growing. The hyphae secrete digestive enzymes which break down the substrate, enabling the fungus to absorb the nutrients contained within the substrate. There are four major groups of fungi each containing aquatic species: Zygomycota (the True Fungi), Ascomycota (the Sac Fungi), Basidiomycota (the Club Fungi), and Deuteromycota (the Fungi Imperfecti).