Fungi are heterotrophs that feed by absorption. Fungi do not eat their food. Instead, a fungus absorbs nutrients from the environment outside its body. These enzymes break down complex molecules to smaller organic compounds that the fungi can absorb into their surroundings. Other fungi use enzymes to penetrate the walls of plant cells, enabling the fungi to absorb nutrients from the plant cell. The most common fungal body structures are multicellular filaments and single cells. The bodies of fungi are usually from a network of tiny filaments called hyphae. Unlike plant cells, the cell walls of fungi are composed of chitin. This material is also found in the exoskeleton of insects and arthropods. Fungal hyphae form an interwoven mass called mycelium that infiltrates the material on which the fungus feeds. In most fungi, the hyphae are divided into cells by cross-walls called speta. The fungi that lack septa are ceonocytic fungi. Some fungi have specialized hyphae that allow them to feed on living animals. Other fungal species have specialized hyphae called haustroria, which fungi use to extract nutrients from or with their hosts. Mutual relationships between fungi and plant roots are called mycorrihizae. Their are two types of mycorrihizae fungi: Ectomycorrhizal and Arbuscular fungi.