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A chaeta or cheta (; ) is a
Both lobes of the parapodium, the notopodium and neuropodium can bear chaetae. Chaetae on the notopodium are called notochaetae and those on the neuropodium are called neurochaetae. Thick, internal chaetae that provide support for well-developed notopodia or parapodia are called
Lepidonotus F166815 neurochaetae.png, unidentate neurochaetae from a ''Lepidonotus'' sp. specimen from Museums Victoria
Lepidonotus neuro F64528.jpg, bidentate neurochaetae from a ''Lepidonotus'' sp. specimen from Museums Victoria
A chaeta or cheta (; ) is a chitin
Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
ous bristle or seta
In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms.
Animal setae
Protostomes
Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...
found on annelid
The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...
worms, although the term is also frequently used to describe similar structures in other invertebrates such as arthropod
Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s. Polychaete annelids (''polychaeta'' literally meaning "many bristles") are named for their chaetae. In Polychaeta, chaetae are found as bundles on the parapodia
In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; : parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed late ...
, paired appendages
An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...
on the side of the body. The chaetae are epidermal
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...
, extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
structures, and clearly visible in most polychaetes. They are probably the best-studied structures in these animals. Segments bearing chaetae are called chaetigers.
Use in taxonomy and identification
The ultrastructure of chaetae is fundamentally similar for all taxa but there is vast diversity in chaetal morphology. Moreover, chaetae bear precise characters for determination of species and taxonomic assessment. The shape, absolute and relative size, number, position, ornamentation and type are important taxonomic characters and specific types are often associated with families or genera. They are sometimes also species-specific and in some cases can be used to differentiate otherwise identical-looking species.Types of chaetae
Both lobes of the parapodium, the notopodium and neuropodium can bear chaetae. Chaetae on the notopodium are called notochaetae and those on the neuropodium are called neurochaetae. Thick, internal chaetae that provide support for well-developed notopodia or parapodia are called acicula
Acicula (: aciculum) are strong, stout internal chaeta
A chaeta or cheta (; ) is a chitinous bristle or seta found on annelid worms, although the term is also frequently used to describe similar structures in other invertebrates such as art ...
.
A wide range of chaetal shapes and arrangements exists:
Basic forms are capillaries and spines. Capillaries are the most common form of chaetae and are very thin and tapering. Spines are also common but are thicker and stouter than capillaries and may be curved or straight and can be distally dentate (e.g. unidentate or bidentate).
Furcate (also called comb, forked or brush) chaetae are similar to capillaries for most of their length but expand distally into a flattened comb- or fork-like tip.
Hooks are stout chaetae that curve distally and may be dentate or strongly curved (falcate). These chaetae may also be capped with a translucent hyaline
A hyaline substance is one with a glassy appearance. The word is derived from , and .
Histopathology
Hyaline cartilage is named after its glassy appearance on fresh gross pathology. On light microscopy of H&E stained slides, the extracellula ...
hood.
Compound chaetae possess a joint or hinge toward the distal end that allows a distal blade or appendage to articulate around it.
Uncini are highly modified chaetae in which the shaft is reduced (or virtually absent) and the tip is broadened to create short, stout structures. These are often present in sedentary taxa.
See also
*Chaetotaxy
Chaetotaxy is the arrangement of bristles (macrochaetae) on an arthropod or annelid, or taxonomy based on their position and size. For example, it is important in Diptera, in which group it was formalised by Ernst August Girschner. The term chaet ...
References
{{reflist Annelid anatomy