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Entoprocta

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Entoprocta

Entoprocts
Temporal range: Early Cambrian–Recent
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Barentsia discreta
Scientific classification
Kingdom: Animalia
Superphylum: Lophotrochozoa
Phylum: Entoprocta
Nitsche, 1870
Families

Entoprocta, whose name means "anus inside", is a phylum of mostly sessile aquatic animals, ranging from 0.1 to 7 millimetres (0.0039 to 0.2756 in) long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that draw food particles 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 larvae swim for a short time and then settle on a surface. There they metamorphose, 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 stolons.

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 molluscs and annelids. However, a study in 2008 concluded that entoprocts are closely related to bryozoans. Recently, the Maotianshan Shales fossil, Cotyledion tylodes, has been reevaluated as being an ancient, sclerite-bearing entoproct (originally having been identified as a putative carpoid echinoderm).[1] This entoproct interpretation of Cotyledion, however, has been questioned by Mark McMenamin, who considers it best interpreted as a stem group echinoderm based on the morphology of its stem sclerites.[3]

Contents

  • Names 1
  • Description 2
    • Distinguishing features 2.1
    • Zooids 2.2
  • Feeding, digestion, excretion, circulation and respiration 3
  • Reproduction and life cycle 4
  • Taxonomy 5
  • Evolutionary history 6
    • Fossil record 6.1
    • Family tree 6.2
  • Ecology 7
    • Distribution and habitats 7.1
    • Interaction with other organisms 7.2
  • References 8
  • Further reading 9
  • External links 10

Names

"Entoprocta", coined in 1870,[4] means "anus inside".[5] The alternative name "Kamptozoa", meaning "bent" or "curved" animals,[6] was assigned in 1929.[4] Some authors use "Entoprocta",[7][8] while others prefer "Kamptozoa".[5][9]

Description

Most species are colonial, and their members are known as "zooids",[5] since they are not fully independent animals.[10] Zooids are typically 1 millimetre (0.039 in) long but range from 0.1 to 7 millimetres (0.0039 to 0.2756 in) long.[5]

Distinguishing features

Entoprocts are superficially like bryozoans (ectoprocts), as both groups have a "crown" of tentacles whose cilia generate water currents that draw food particles towards the mouth. However, they have different feeding mechanisms and internal anatomy, and ectoprocts undergo a metamorphosis from larva to adult that destroys most of the larval tissues; their colonies also have a founder zooid which is different from its "daughters".[5][5]

Summary of distinguishing features
  Entoprocta[5] Bryozoa (Ectoprocta)[5]
Tentacles Solid Hollow
Feeding current From bases to tips of tentacles From tips to bases of tentacles
Position of anus Inside "crown" of tentacles Outside "crown" of tentacles
Coelom none Three-part
Shape of founder zooid in a colony Same as other zooids Round, unlike normal zooids[11]
Metamorphosis to adult Retains most larval structures Destroys most larval structures
Excretory organs Protonephridia None

Zooids

Barentsa discreta

The body of a mature entoproct zooid has a goblet-like structure with a calyx mounted on a relatively long stalk that attaches to a surface. The rim of the calyx bears a "crown" of 8 to 30 solid tentacles, which are extensions of the body wall. The base of the "crown" of tentacles is surrounded by a membrane that partially covers the tentacles when they retract. The mouth and anus lie on opposite sides of the atrium (space enclosed by the "crown" of tentacles), and both can be closed by sphincter muscles. The gut is U-shaped, curving down towards the base of the calyx, where it broadens to form the stomach. This is lined with a membrane consisting of a single layer of cells, each of which has multiple cilia.[5]

Pedicellina cernua (magnified x 27)

The stalks of colonial species arise from shared attachment plates or from a network of stolons, tubes that run across a surface.[5] In solitary species, the stalk ends in a muscular sucker, or a flexible foot, or is cemented to a surface.[8] The stalk is muscular and produces a characteristic nodding motion. In some species it is segmented. Some solitary species can move, either by creeping on the muscular foot or by somersaulting.[5]

The body wall consists of the epidermis and an external cuticle,[5] 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.[5] The stolons and stalks of colonial species have thicker cuticles, stiffened with chitin.[8]

There is no

  • Information on Entoprocta from earthlife.net

External links

Further reading

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  5. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab
  6. ^ The prefix "campto-" is explained at:
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  8. ^ a b c d e f g h i
  9. ^ a b c d e f
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  24. ^ a b
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  26. ^ Sid Perkins, "ScienceShot: Fossils of Enigmatic Sea Creature Emerge", ScienceNOW, January 17, 2013
  27. ^ a b c
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References

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 invader, possibly as a result of human activities.[34]

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.[32] In the White Sea, the non-colonial entoproct Loxosomella nordgaardi prefers to live attached to bryozoan (ectoproct) colonies, mainly on the edges of colonies or in the "chimneys", gaps by which large bryozoan colonies expel water from which they have seived 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.[33]

Some species of nudibranchs ("sea slugs"), particularly those of the genus Trapania, as well as turbellarian flatworms, prey on entoprocts.[31]

Interaction with other organisms

All species are sessile.[5] While the great majority are marine, two species live in freshwater: Loxosomatoides sirindhornae, reported in 2004 in central Thailand, and Urnatella gracilis, found in all the continents except Antarctica.[4] Colonial species are found in all the oceans, living on rocks, shells, algae and underwater buildings.[5] The solitary species, which are marine,[4] live on other animals that feed by producing water currents, such as sponges, ectoprocts and sessile annelids.[8]

Distribution and habitats

Ecology

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 molluscs, annelids, flatworms, nemertines and others. However, scientists disagree about which phylum is mostly closely related to enctoprocts within the trochozoans.[30] 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.[27]

[29][27] as a name for the currently identified bryozoans.Ectoprocta, and resurrecting class (biology) However, studies by one team in 2007 and 2008 argue for sinking Entoprocta into Bryozoa as a [28] When entoprocts were discovered in the nineteenth century, they and bryozoans (ectoprocts) were regarded as classes within the phylum

Family tree

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.[26]

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.[25]

Since entoprocts are small and soft-bodied, fossils have been extremely rare.[25] In 1977, [24]

The Mid-[24] but the classification is uncertain[25]

Fossil record

Evolutionary history

Family Barentsiidae Pedicellinidae Loxokalypodidae Loxosomatidae
Genera Barentsia, Coriella, Pedicellinopsis, Pseudopedicellina, Urnatella[20] Chitaspis, Loxosomatoides, Myosoma, Pedicellina[21] Loxokalypus[22] Loxocore, Loxomitra, Loxosoma, Loxosomella, Loxosomespilon[23]
Colonial[9] Colonial Solitary
Septum between calyx and stalk[9] Yes No
Star-cell organ[9] Yes No
Anus on cone[5] No Yes
Stolons present[9] Yes No, colonies grow on shared baseplate Not colonial
Segmented stems[5][9] Yes No

The phylum consists of about 150 recognized species, grouped into 4 families:[5][7]

Taxonomy

[8] All species can produce

[5] In some species the larva is a

[5] at any stage.coelom There is no [18].embryo labelled "4d" in the early cell develops from a specific mesoderm, and spiral cleavage pattern: the cells divide by spiralian The development of the fertilized egg into a larva follows a typical [5].yolkMost species are simultaneous

Apical tuft (cilia)
Prototroch (cilia)
Stomach
Mouth
Metatroch (cilia)
Mesoderm
Anus
/// = cilia
Trochophore larva[17]

Reproduction and life cycle

The zooids absorb oxygen and emit carbon dioxide by diffusion,[5] which works well for small animals.[15]

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.[5][14] 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.[5]

In addition, glands in the tentacles secrete sticky threads that capture large particles.[5] A non-colonial species reported from around the Antarctic Peninsula in 1993 has cells that superficially resemble the cnidocytes of cnidaria, and fire sticky threads. These unusual cells lie around the mouth, and may provide an additional means of capturing prey.[13]

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 "seive"); and downstream collecting, in which food articles 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.[12]

A band of cells, each with multiple cilia, runs along the sides of the tentacles, connecting each tentacle to its neighbors, except that there is a gap in the band nearest the anus. A separate band of cilia grows along a groove that runs close to the inner side of the base of the "crown", with a narrow extension up the inner surface of each tentacle.[8] The cilia on the sides of the tentacles create a current that flows into the "crown" at the bases of the tentacles and exits above the center of the "crown".[5] These cilia pass food particles to the cilia on the inner surface of the tentacles, and the inner cillia produce a downward current that drives particles into and around the groove, and then to the mouth.[8]

Feeding, digestion, excretion, circulation and respiration

[5]

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