Entoprocta is a phylum of tiny, aquatic, largely marine, filter-feeding invertebrates attached to the substratum with a slender stalk and characterized by having tentacles with a downstream-collecting ciliary system, an anus inside the ring of ciliated tentacles, and no coelomic canal. Entoprocta are generally vase shaped. They range in size from only about 0.5 millimeters to 5.0 millimeters (0.02-0.2 inches). They are known as entoprocts, goblet worms, and kamptozoans.
The phylum includes about 170 species in four families. While most species are marine, there is one freshwater genus, Urnatella, and the freshwater species Urnatella gracilis is widespread.
Entoprocts display a bi-level functionality in aquatic food chains, securing nutrition essential for their own needs by capturing phytoplankton, other small organisms, and organic particles with their tentacles by creating a water current with the cilia along their tentacles, while also providing nutrition to the various animals, such as fish and sea urchins, that feed on them.
Entoprocts were originally grouped together with the ectoprocts in the phylum Bryozoa. Both entoprocts and ectoprocts are tiny, aquatic, sessile, filter feeding organisms characterized by a "crown" of ciliated tentacles used for filter feeding. This tentacle "crown"—known as a lophophore and characteristic also of the ectoprocts (as well members of Phoronida and Brachiopoda)—is essentially a tentacle-bearing ribbon or string that is an extension (either horseshoe-shaped or circular) surrounding the mouth (Smithsonian 2007; Luria et al. 1981). Despite being similar in terms of their lophophores, however, the entoprocts and ectoprocts are very distinct and unrelated phyla (Thorp and Covich 2001).
One notable distinction between the Endoprocta and the Ectoprocta is that the ectoprocts have their anus outside their ring of tentacles, whereas the endoprocts have their anus inside the space enclosed by their tentacles (Ramel 2008). Indeed, the ecotoprocts are classified taxonomically together with the phoronids and brachiopods because of the lophophore, which may be defined as a crown of ciliated mesosomal tentacles surrounding the mouth but not the anus (Thorp and Covich 2001). Some, however, define lophophore more generally as a tentacle crown of ciliated tentacles and include Entoprocta as having a lophophore, but with the anus inside or on the lophophore (Visser and Veldhuijzen van Zanten 2003). The name Entoprocta comes from the Greek εντος, entos, meaning "inside," and προκτος, proktos, meaning "anus."
Another distinction between the ectoprocts and endoprocts is that the lophophore of ectoprocts has an upstream-collecting ciliary band, whereas the endoprocts have an downstream-collecting ciliary system like trochophore larvae and adult rotifers (Nielsen 2002). Furthermore, the Ectoprocta are coelomate (possessing a body cavity) and their embryos undergo radial cleavage, while the Entoprocta are acoelemate and undergo spiral cleavage. Entoprocts are protostomes, whose coelom lining is formed by the mesoderm after the cells of the developing embryo split at the junction of the endoderm and ectoderm during gastrulation and there is rapid division of cells (Towle 1989).
Molecular studies are ambiguous about the exact taxonomic position of the Entoprocta, but they do make it clear that Entoprocta do not have a close relationship with the Ectoprocta. For these reasons, the Entoprocta are now considered a phylum of their own (Valentine 2004).
The entoprocts typically have a U-shaped gut (digestive function), a ganglion (reflex function), a pair of protonephridia (metabolic waste removal function), a pair of gonads (reproductive function), and a tentacular crown (food acquisition function), with both the mouth and anus inside the crown, and the main body, or calyx, supported by a slender stalk that attaches to the substratrum (Iseto 2004).
Entoprocts are filter feeders. The tentacles are ciliated, and the beating of the cilia creates a current of water that drives water, together with entrained food particles (mainly phytoplankton), toward the tentacles. The tentacles secrete a mucus that catches food particles, which are then moved toward the mouth, through grooves, by the cilia on the tentacles. The tentacles cannot be retracted within the zooecium, but can be folded into a central depression (Visser and Veldhuijzen van Zanten 2003). Entoprocts feed on small microorganisms, diatoms, algae, and organic particles.
Some species are colonial, with multiple animals on branching systems of stalks. The adult loxosomatids (family Loxosomatidae) are solitary.
Nearly all species are sedentary, being attached to the substrate by a stalk, with the body being cup-shaped. The zooids (individual, distinct organisms) may be on branched or unbranched stalks, with the stalks bending at the muscular urn-shaped segments, limiting movements (Visser and Veldhuijzen van Zanten 2003). The phylum also has been called Kamptozoa, however, meaning "bending animal," because they do very actively move (Iseto 2004). Some solitary species can glide over the ground, similar to slugs, and one species is known to walk using a "foot" with leglike extensions (Iseto 2004).
Entoprocts can reproduce either by budding, or sexually. Each zooid of a colonial species is generally male or female, but with both sexes in a colony. Solitary species are generally sequential hermaphrodites that start with male reproductive organs but change to having female reproductive organs (protandrous hermaphrodites). Eggs are fertilized in the ovary (Iseto 2004).
The larva is a typical trochophore (ciliated, free-swimming planktonic marine larva) with an apical organ ("front" end sense organ). Some species of Loxosomella and Loxosoma have larvae with a long-lasting planktontrophic stage, while most other entoproct species have a short, free-swimming larval stage and the larva can settle shortly after liberation (Thorp and Covich 2001). The apical organ is lost after metamorphosis and a new "brain" develops. The adult feeding structures, with tentacles with a downstream-collecting ciliary system, develops anew after metamorphosis.
The four families of Entoprocta are all marine, with the exception of the freshwater genus Urnatella in the predominately marine family Barentsiidae (Visser and Veldhuijzen van Zanten 2003). The species are widely distributed, including in tropical, temperate, and polar marine waters, and from shallow seashore to deep sees (below 500 meters, 1640 feet) (Iseto 2004). The freshwater genus Urnatella has been reported in the United States and from India to central Europe (Visser and Veldhuijzen van Zanten 2003).
Colonial species are found on diverse substrata, including rocks, stones, shells, and other animals, while most solitary species live on the bodies of host animals, such as sponges, polychaetes, and bryozoans (Iseto 2004).
The relationships of entoprocts to other invertebrates are obscure but some consider they may have affinities to the spiralians, which are invertebrates that show spiral cleavage patterns (Iseto 2004; Thorp and Covich 2001). The entoprocts are placed into four families and fifteen or sixteen genera. The Loxosomatidae comprise all of the solitary species while the other four families comprise colonial species. Among the colonial families, the members of Barentsiidae have a muscular swelling at the base of the stalk; members of Pedicellinidae lack the basal muscular swelling and each zooid of a colony is interrupted by a stolon; members of Loxokalypodidae lack the basal muscular swelling, and component zooids of a colony arise from a common basal plate not interrupted by stolons (Iseto 2004).
Family Barentsiidae (Emschermann, 1972)
Family Loxokalypodidae (Emschermann, 1972)
Family Loxosomatidae (Hincks, 1880)
Family Pedicellinidae (Johnston, 1847)
The only [[fossil] of Entoprocta traces to the Upper Jurasic and is placed in the extant genus Barentsia (Iseto 2004).
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