Phylum Cyclorhagida and Homalorhagida, with 21 genera and over 200 species, some of which have only been described from juvenile stages. These animals are very small, generally less than 1 mm in length, segmented, and limbless. They are motile, marine (occasionally brackish), benthic invertebrates found worldwide, from intertidal to abyssal zones, on sediment layers, sandy beaches, in algae, or associated with other, larger invertebrates. Their bodies are covered with multiple spines, which are movable extensions of the body wall. The spines around the head are used to help pull these animals along the benthos, where they deposit feed on diatoms, bacteria, and other organic matter in the substrate. The head and pharynx can be retracted and covered with plates (called placids) on the second body segment, differences in the number and closing mechanisms of these plates are often used to distinguish species and higher taxonomic groups. (Brusca and Brusca, 1990; Nehaus, 2012; Neuhas and Higgins, 2002; Schmidt-Rhaesa, 2012; Zhang, 2011)is divided into two orders,
Species belonging to this phylum have been described from marine and brackish waters around the world. They have been found at depths from 8 to 8,000 meters. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)
The evolutionary relationships within phylum loriciferans and priapulids, with whom they form superphylum Scalidophora. The inter-relationships of these phyla are still a matter of debate (although the majority of evidence points to a sister relationship between kinorhynchs and priapulids), as is the membership of this clade in the group Ecdysozoa (molting animals). The latter ambiguity is due to inadequate sampling of scalidophoran species in molecular examinations of possible ecdysozoans, as well as the lack of evidence that ecdysteroid hormones induce molting in this and other groups. (Aleshin, et al., 1998; Halanych, 2004; Nehaus, 2012)are still poorly resolved. Kinorhynchs are thought to be most closely related to
Some authors have additionally included nematomorphans as evolutionary relatives to scalidophoran phyla, in a clade called Cephaloryncha. Genetic evidence has provided support for a sister relationship between loriciferans and nematomorphans, which would render Scalidophora paraphyletic and support the future use of the name Cephaloryncha, as modern taxonomic systems of classification endeavor to give names only to monophyletic groups. (Aleshin, et al., 1998; Halanych, 2004; Sørensen, et al., 2008)
Underneath the tergal and sternal plates, kinorhynchs have an epidermis which is non-ciliated and closely associated with the nervous system. Bands of dorsolateral and ventrolateral intersegmental, cross-striated muscles attach to the cuticle; strong fibers connect the sternal and tergal plates. Kinorhynchs have head retractor muscles as well as head cone retractor muscles, stretching between the base of the head scalids/mouth cone and posterior trunk segments. Additionally, there are circular muscles associated with the neck and mouth cone placids, but no circular body wall muscles. These animals are pseudocoelemates, and have a spacious anterior body cavity to allow for head retraction, their digestive and reproductive organs exist within blastocoelomic spaces. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Neuhas and Higgins, 2002)
The mouth leads into a buccal cavity that is followed by a sucking pharynx and esophagus, all of which are lined with cuticle. The esophagus is connected to a long, straight midgut, which is made of over 95% epithelial cells; these are interlocked and presumptively absorb nutrients. The midgut leads to a short hindgut, also lined with cuticle, terminating in an anus on the last zonite. Kinorhynchs excrete wastes via a pair of protonephridia located in the 8th and 9th trunk zonites, dorsolateral to the gut. These open through pores in the body cuticle and may have ciliate cells and/or microvilli. Gas diffusion takes place across the body wall and circulates by diffusion through the body cavity. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Ruppert, et al., 2004)
Each ovary produces one large oocyte, fertilization is assumed to occur internally. Fertilized eggs are deposited in egg cases and embryos undergo direct development to juvenile stages, emerging with 11 of their 13 body segments fully formed. The final two segments, as well as a number of head scalids, are added during a series of six molts, as kinorhynchs progress to adulthood. Molting has only been observed in adults of two species. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)
Both males and females have paired gonads and short gonoducts. Males have two or three hollow, often rigid, spicules associated with their gonopores. When present, placement of these spicules varies from species to species. Females have both germ cells and nutritive cells in their gonads, and oviducts with seminal receptacles; their gonopores open between the 12th and 13th zonites. Females have been found with a brownish mucosal mass comprised of spermatozoa and spermatids, which is considered to be a spermatophore surrounding their posteriors. Mature spermatozoa may be as large as 1/4 of the total body length of the adult male. Mating has only been observed in one species, Pycnophyes kielensis. In this case, a male and female positioned themselves so their ventral posterior ends faced each other, with the mucosal mass surrounding their posterior ends. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)
Little is known about egg production and early development in this group. Females provide yolk and an egg case for their eggs. Further parental investment is unknown. (Brusca and Brusca, 1990)
There is currently no information available regarding the typical lifespan of (Brusca and Brusca, 1990).
These animals have simple nervous systems, with a series of ten ganglia connected and arranged in a ring around the pharynx. The epithelium is highly innervated. Scalids contain up to 10 monociliary sensory cells, which are connected to an exterior distal pore; these are likely chemo-and mechanoreceptive. All species in this phylum have species-specific patterns of sensory spots on their trunks, consisting of one or two pores surrounded by micropapillae. These sensory spots may have a cilium. Microvillar eyespots may be located on the pharyngeal nerve ring and some species have red-pigmented, light sensitive ocelli behind the mouth cone. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Neuhas and Higgins, 2002)
There are no known positive effects ofon humans beyond the place they occupy in the marine food chain and their use in scientific research.
There are no known adverse effects ofon humans.
As a broadly cosmopolitan phylum,in general are not considered endangered or threatened, nor are there any conservation efforts directed at any individual species.
Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
lives on Antarctica, the southernmost continent which sits astride the southern pole.
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
areas with salty water, usually in coastal marshes and estuaries.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
active at dawn and dusk
an animal that mainly eats decomposed plants and/or animals
union of egg and spermatozoan
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an animal that mainly eats plankton
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.
mainly lives in oceans, seas, or other bodies of salt water.
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
breeding takes place throughout the year
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