The peniculids are an order of
ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
protozoa
Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...
, including the well-known ''
Paramecium
''Paramecium'' ( , , plural "paramecia" only when used as a Common name, vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and Ocean, marine environments. Paramecia are often abundant in stagna ...
'' and related genera, such as ''
Frontonia
''Frontonia'' is a genus of free-living unicellular ciliate protists, belonging to the order Peniculida. As Peniculids, the ''Frontonia'' are closely related to members of the genus ''Paramecium''. However, whereas ''Paramecia'' are mainly ba ...
'', ''
Stokesia'', ''
Urocentrum'' and ''
Lembadion''. Most are relatively large, freshwater forms that feed by sweeping smaller organisms into the mouth. They have simple life cycles, and in many cases do not even form
resting cysts.
Typically the body has uniform, dense
cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
, which also cover a vestibule preceding the mouth.
Extrusome
Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extruso ...
s are characteristically in the form of spindle trichocysts, which release thread-like shafts, and never mucocysts. The oral cilia include peniculi, corresponding to the
membranelle Membranelles (also membranellae) are structures found around the mouth, or cytostome, in ciliates. They are typically arranged in series, to form an "adoral zone of membranelles", or AZM, on the left side of the buccal cavity (peristome). The membra ...
s of related groups, arranged parallel to the mouth deep in the oral cavity. Nematodesmata (rods) arise from the bases of the oral or perioral cilia, but these do not support a