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Entoprocta (), or Kamptozoa , is a phylum of mostly sessile aquatic , ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that filter feeder towards the mouth, and both the mouth and anus lie inside the "crown". The superficially similar Bryozoa (Ectoprocta) have the anus outside a "crown" of hollow tentacles. Most families of entoprocts are colonial, and all but 2 of the 150 species are marine. A few solitary species can move slowly.
Some species eject unfertilized ovum into the water, while others keep their ova in brood chambers until they hatch, and some of these species use placenta-like organs to nourish the developing eggs. After hatching, the swim for a short time and then settle on a surface. There they Metamorphosis, and the larval gut rotates by up to 180°, so that the mouth and anus face upwards. Both colonial and solitary species also reproduce by cloning — solitary species grow clones in the space between the tentacles and then release them when developed, while colonial ones produce new members from the stalks or from corridor-like .
Fossils of entoprocts are very rare, and the earliest specimens that have been identified with confidence date from the Late Jurassic. Most studies from 1996 onwards have regarded entoprocts as members of the Trochozoa, which also includes and . However, a study in 2008 concluded that entoprocts are closely related to bryozoans. Other studies place them in a clade Tetraneuralia, together with molluscs.
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The body wall consists of the epidermis and an external cuticle, which consists mainly of criss-cross collagen fibers. The epidermis contains only a single layer of cells, each of which bears multiple cilia ("hairs") and microvilli (tiny "pleats") that penetrate through the cuticle. The stolons and stalks of colonial species have thicker cuticles, stiffened with chitin.
There is no coelom (internal fluid-filled cavity lined with peritoneum) and the other internal organs are embedded in connective tissue that lies between the stomach and the base of the "crown" of tentacles. The nervous system runs through the connective tissue and just below the epidermis, and is controlled by a pair of ganglia. Nerves run from these to the calyx, tentacles and stalk, and to sense organs in all these areas.
Entoprocts generally use one or both of: ciliary sieving, in which one band of cilia creates the feeding current and another traps food particles (the "sieve"); and downstream collecting, in which food particles are trapped as they are about to exit past them. In entoprocts, downstream collecting is carried out by the same bands of cilia that generate the current; trochozoan larvae also use downstream collecting, but use a separate set of cilia to trap food particles.
In addition, glands in the tentacles secrete sticky threads that capture large particles. A non-colonial species reported from around the Antarctic Peninsula in 1993 has cells that superficially resemble the of cnidaria, and fire sticky threads. These unusual cells lie around the mouth, and may provide an additional means of capturing prey.
The stomach and intestine are lined with microvilli, which are thought to absorb nutrients. The anus, which opens inside the "crown", ejects solid wastes into the outgoing current after the tentacles have filtered food out of the water; in some families it is raised on a cone above the level of the groove that conducts food to the mouth. Most species have a pair of protonephridia which extract soluble wastes from the internal fluids and eliminate them through pores near the mouth. However, the freshwater species Urnatella gracilis has multiple nephridia in the calyx and stalk.
The zooids absorb oxygen and emit carbon dioxide by diffusion, which works well for small animals.
In some species the larva is a trochophore which is and feeds on floating food particles by using the two bands of cilia round its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. In some species of the genus Loxosomella and Loxosoma, the larva produces one or two buds that separate and form new individuals, while the trochophore disintegrates. However, most produce a larva with sensory tufts at the top and front, a pair of pigment-cup ocelli ("little eyes"), a pair of protonephridia, and a large, cilia-bearing foot at the bottom. After settling, the foot and frontal tuft attach to the surface. Larvae of most species undergo a complex metamorphosis, and the internal organs may rotate by up to 180°, so that the mouth and anus both point upwards.
All species can produce cloning by budding. Colonial species produce new zooids from the stolon or from the stalks, and can form large colonies in this way. In solitary species, clones form on the floor of the atrium, and are released when their organs are developed.
In 1992 J.A. Todd and P.D. Taylor concluded that Dinomischus was not an entoproct, because it did not have the typical rounded, flexible tentacles, and the fossils showed no other features that clearly resembled those of entoprocts. In their opinion, the earliest fossil entoprocts were specimens they found from Late Jurassic rocks in England. These resemble the modern colonial genus Barentsia in many ways, including: upright zooids linked by a network of stolons encrusting the surface to which the colony is attached; straight stalks joined to the stolons by bulky sockets with transverse bands of wrinkles; overall size and proportions similar to that of modern species of Barentsia.
Another species, Cotyledion tylodes, first described in 1999, was larger than extant entoprocts, reaching 8–56 mm in height, and unlike modern species, was "armored" with sclerites, scale-like structures. C. tylodes did have a similar sessile lifestyle to modern entoprocts. The identified fossils of C. tylodes were found in 520-million-year-old rocks from southern China. This places early entoprocts in the period of the Cambrian explosion.Sid Perkins, "ScienceShot: Fossils of Enigmatic Sea Creature Emerge", ScienceNOW, January 17, 2013
The consensus of studies from 1996 onwards has been that entoprocts are part of the Trochozoa, a protostome "superphylum" whose members are united in having as their most basic larval form the trochophore type. The trochozoa also include , , , and others. However, scientists disagree about which phylum is mostly closely related to enctoprocts within the trochozoans. An analysis in 2008 re-introduced the pre-1869 meaning of the term "Bryozoa", for a group in which entoprocts and ectoprocts are each other's closest relatives.
Small colonies of the freshwater entoproct Urnatella gracilis have been found living on the aquatic larvae of the dobsonfly Corydalus cornutus. The ectoprocts gain a means of dispersal, protection from predators and possibly a source of water that is rich in oxygen and nutrients, as colonies often live next to the gills of the larval flies. In the White Sea, the non-colonial entoproct Loxosomella nordgaardi prefers to live attached to (ectoproct) colonies, mainly on the edges of colonies or in the "chimneys", gaps by which large bryozoan colonies expel water from which they have sieved food. Observation suggests that both the entoprocts and the bryozoans benefit from the association: each enhances the water flow that the other needs for feeding; and the longer cilia of the entoprocts may help them to capture different food from that caught by the bryozoans, so that the animals do not compete for the same food.
Entoprocts are small and have been little studied by zoologists. Hence it is difficult to determine whether a specimen belongs to a species that already occurs in the same area or is an invasive species, possibly as a result of human activities.
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