by a cytoskeletal infrastructure, which may be referred to as a "pellicle". the pellicle gives shape to the cell, especially during locomotion. pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. in ciliates and apicomplexa, the pellicle includes a layer of closely packed vesicles called alveoli. in euglenids, the pellicle is formed from protein strips arranged spirally along the length of the body. familiar examples of protists with a pellicle are the euglenoids and the ciliate paramecium. in some protozoa, the pellicle hosts epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili). some protozoa live within loricas - loose fitting but not fully intact enclosures. for example, many collar flagellates (choanoflagellates) have an organic lorica or a lorica made from silicous sectretions. loricas are also common among some green eugenids, various ciliates (such as the folliculinids, various testate amoebae and foraminifera. the surfaces of a variety of protozoa are covered with a layer of scales and or spicules. examples include the amoeba cochliopodium, many centrohelid heliozoa, synurophytes. the layer is often assumed to have a protective role. in some, such as the actinophryid heliozoa, the scales only form when the organism encysts. the bodies of some protozoa are supported internally by rigid, often inorganic, elements (as in acantharea, pylocystinea, phaeodarea - collectively the 'radiolaria', and ebriida). life cycle protozoa mostly reproduce asexually by binary fission or multiple fission. many protozoa also exchange genetic material by sexual means (typically, through conjugation), but this is generally decoupled from reproduction. meiotic sex is widespread among eukaryotes, and must have originated early in their evolution, as it has been found in many protozoan lineages that diverged early in eukaryotic evolution. ecology free-living free-living protozoa are found in almost all ecosystems that contain free water, permanently or temporarily. they have a critical role in the mobilization of nutrients in ecosystems. within the microbial food web they include the most important bacterivores. in part, they facilitate the transfer of bacterial and algal production to successive trophic levels, but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. as consumers, protozoa prey upon unicellular or filamentous algae, bacteria, microfungi, and micro-carrion. in the context of older ecological models of the micro- and meiofauna, protozoa may be a food source for microinvertebrates. most species of free-living protozoa live in similar habitats in all parts of the world. parasitism main article: protozoan infection further information: list of parasites of humans many protozoan pathogens are human parasites, causing serious diseases such as malaria, giardiasis, toxoplasmosis, and sleeping sickness. some of these protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and resting cysts, enabling them to survive harsh conditions. commensalism a wide range of protozoa live commensally in the rumens of ruminant animals, such as cattle and sheep. these include flagellates, such as trichomonas, and ciliated protozoa, such as isotricha and entodinium. the ciliate subclass astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms. mutualism association between protozoan symbionts and their host organisms can be mutually beneficial. flagellated protozoa such as trichonympha and pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules. trophozoites of entamoeba histolytica, a disease-causing parasite with engulfed red blood cells (dark circles) isotricha intestinalis, a commensal ciliate in the rumen of sheep trichonympha campanula, a mutualist partner of termites