Pygidium
Encyclopedia
The pygidium is the posterior body part or shield of crustacean
s and some other arthropod
s, such as insect
s and the extinct trilobite
s. It contains the anus
and, in females, the ovipositor
. It is composed of fused body segments, sometimes with a tail, and separated from thoracic
segments by an articulation.
s, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in Palpigradi, Amblypygi, Thelyphonida, Schizomida, Ricinulei
and in the extinct order Trigonotarbida
. It is also present in early fossil representatives of horseshoe crabs.
, or spiny, as in order Lichida
. They can be classified into four categories according to their relative size in comparison to the cephalon
.
They can further be subdivided in their morphological similarity to the thorax
. Pygidia that are similar in shape and form to the thoracic segments are termed homonomous, while pygidia that vary significantly from the shape and form of the thoracic segments (like by the presence or absence of spines) are heteronomous.
of fish
of the family
Trichomycteridae
. Most species of this genus have been reassigned to the genus Trichomycterus
.
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s and some other arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s, such as insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s and the extinct trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s. It contains the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
and, in females, the ovipositor
Ovipositor
The ovipositor is an organ used by some animals for oviposition, i.e., the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly...
. It is composed of fused body segments, sometimes with a tail, and separated from thoracic
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
segments by an articulation.
Arachnids
In arachnidArachnid
Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...
s, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in Palpigradi, Amblypygi, Thelyphonida, Schizomida, Ricinulei
Ricinulei
The Order Ricinulei is a group of arachnids known as hooded tickspiders. In older works they are sometimes referred to as Podogona., 60 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. They occur today in west-central Africa and the Neotropical...
and in the extinct order Trigonotarbida
Trigonotarbida
The Order Trigonotarbida is an extinct group of arachnids whose fossil record extends from the late Silurian to the early Permian . These animals are known from several localities in Europe and North America, as well as a single record from Argentina. Trigonotarbids can be envisaged as spider-like...
. It is also present in early fossil representatives of horseshoe crabs.
Trilobites
In trilobites, the pygidium can range from extremely small (much smaller than the head, or cephalon) to larger than the cephalon. They can be smooth, as in order AsaphidaAsaphida
Asaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian to the Silurian. The order contains six superfamilies , but no suborders...
, or spiny, as in order Lichida
Lichida
Lichida is an order of typically spiny trilobite that lived from the Ordovician to the Devonian period.-Morphology:These trilobites usually have 8-13 thoracic segments. Their exoskeletons often have a grainy texture or have tubercles...
. They can be classified into four categories according to their relative size in comparison to the cephalon
Cephalon
Cephalon, Inc. is a U.S. biopharmaceutical company co-founded in 1987 by Dr. Frank Baldino, Jr., a pharmacologist and former scientist with the DuPont Company, who served as the company's chairman and chief executive officer until his death in December 2010...
.
- Micropygous - the pygidium is considerably smaller than the cephalon.
- Subisopygous - the pygidium is slightly smaller than the cephalon.
- Isopygous - the cephalon and the pygidium are more or less of equal size.
- Macropygous - the pygidium is larger than the cephalon.
They can further be subdivided in their morphological similarity to the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
. Pygidia that are similar in shape and form to the thoracic segments are termed homonomous, while pygidia that vary significantly from the shape and form of the thoracic segments (like by the presence or absence of spines) are heteronomous.
Other uses
Pygidium is also a superseded genusGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Trichomycteridae
Trichomycteridae
Trichomycteridae is a family of catfishes commonly known as the pencil or parasitic catfishes. This family includes the infamous candiru fish, feared by some people for its alleged habit of entering into the urethra of humans....
. Most species of this genus have been reassigned to the genus Trichomycterus
Trichomycterus
Trichomycterus is a genus of catfishes, the largest genus in the family Trichomycteridae with over 100 species currently described.-Taxonomy:This genus is defined by the lack of specializations found in other trichomycterids and is certainly polyphyletic...
.