Entoprocta
Encyclopedia
Entoprocta, whose name means "anus
inside", is a phylum of mostly sessile
aquatic animal
s, ranging from 0.1 to 7 mm (0.00393700787401575 to 0.275590551181102 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 ova
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 larva
e swim for a short time and then settle on a surface. There they metamorphose
, and the larval gut generally 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 stolon
s.
Some species of nudibranch
s ("sea slugs") and turbellarian flatworm
s prey on entoprocts. A few entoproct species have been found living in close association with other animals. It is uncertain whether any are invasive species
.
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 annelid
s. However, a study in 2008 concluded that entoprocts are closely related to bryozoans.
inside". The alternative name "Kamptozoa", meaning "bent" or "curved" animals, was assigned in 1929. Some authors use "Entoprocta", while others prefer "Kamptozoa".
ns (ectoproct)s, 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, and their colonies also have a founder zooid that is different from its "daughters".
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.
The stalks of colonial species arise from shared attachment plates or from a network of stolon
s, tubes that run across a surface. In solitary species the stalk ends in a muscular sucker, or a flexible foot, or is cemented to a surface. 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 somersault
ing.
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 "seive"); and downstream collecting, in which food articles are trapped as they are about to exit past them. In entoprocts, downstream collecting is done 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 cnidocyte
s 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.
s, but some switch from males to females as they mature, while individuals of some species remain of the same sex all their lives. Individuals have one or two pairs of gonad
s, placed between the atrium and stomach, and opening into a single gonopore
in the atrium. The eggs are thought to be fertilized in the ovaries
. Most species release eggs that hatch into planktonic larva
e, but a few brood their eggs in the gonopore. Those that brood small eggs nourish them by a placenta
-like organ, while larvae of species with larger eggs live on stored yolk. The development of the fertilized egg into a larva follows a typical spiralian pattern: the cells divide by spiral cleavage, and mesoderm
develops from a specific cell
labelled "4d" in the early embryo
. There is no coelom
at any stage.
In some species the larva is a trochophore
which is plankton
ic 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 genera
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 point upwards.
All species can produce clone
s 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.
:
and Urnatella gracilis found in all the continents except Antarctica. Colonial species are found in all the oceans, living on rocks, shells, algae and underwater buildings. The solitary species, which are marine, live on other animals that feed by producing water currents, such as sponges, ectoprocts and sessile annelid
s.
s ("sea slugs") and turbellarian flatworm
s prey on entoprocts.
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 bryozoa
n (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.
Entoprocts are small and have been little studied by zoologists. Hence it is hard 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.
Since entoprocts are small and soft-bodied, fossils have been extremely rare. In 1977 Simon Conway Morris
provided the first description of Dinomischus
, a sessile animal with calyx, stalk and holdfast, found in Canada
's Burgess Shale
, which was formed about . Morris regarded this animal as the earliest known entoproct, since its mouth and anus lay inside a ring of structures above the calyx, but noted that these structures were flat and rather stiff, while the tentacles of modern entoprocts are flexible and have a round cross-section.
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.
, because both groups were sessile
animals that filter-fed
by means of a "crown" of tentacles that bore cilia. However, from 1869 onwards increasing awareness of differences, including the position of the entoproct anus
inside the feeding structure and the difference in the early pattern of division
of cells in their embryo
s, caused scientists to regard the two groups as separate phyla
. "Bryozoa" then became just an alternative name for ectoprocts, in which the anus is outside the feeding organ. However studies by one team in 2007 and 2008 argues for sinking Entoprocta into Bryozoa as a class
, and resurrecting Ectoprocta as a name for the currently identified bryozoans.
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, annelid
s, flatworm
s, nemertines 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.
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,...
inside", is a phylum of mostly sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
aquatic animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s, ranging from 0.1 to 7 mm (0.00393700787401575 to 0.275590551181102 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
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
towards the mouth, and both the mouth and anus lie inside the "crown". The superficially similar Bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
(Ectoprocta) have the anus outside a "crown" of hollow tentacles. Most families
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...
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 ova
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
into the water, while others keep their ova in brood chambers until they hatch, and some of these species use placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
-like organs to nourish the developing eggs. After hatching, the larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e swim for a short time and then settle on a surface. There they metamorphose
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
, and the larval gut generally rotates by up to 180°, so that the mouth and anus face upwards. Both colonial and solitary species also reproduce by cloning
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
– 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 stolon
Stolon
In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...
s.
Some species of nudibranch
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
s ("sea slugs") and turbellarian flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s prey on entoprocts. A few entoproct species have been found living in close association with other animals. It is uncertain whether any are invasive species
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
.
Fossils of entoprocts are very rare, and the earliest specimens that have been identified with confidence date from the Late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
. Most studies from 1996 onwards have regarded entoprocts as members of the Trochozoa, which also includes molluscs and annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s. However, a study in 2008 concluded that entoprocts are closely related to bryozoans.
Names
"Entoprocta", coined in 1870, means "anusAnus
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,...
inside". The alternative name "Kamptozoa", meaning "bent" or "curved" animals, was assigned in 1929. Some authors use "Entoprocta", while others prefer "Kamptozoa".
Description
Most species are colonial, and their members are known as "zooids", since they are not fully independent animals. Zooids are typically 1 millimetre (0.0393700787401575 in) long but ranging from 0.1 to 7 mm (0.00393700787401575 to 0.275590551181102 in) long.Distinguishing features
Entoprocts are superficially like bryozoaBryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
ns (ectoproct)s, 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
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
from larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
to adult that destroys most of the larval tissues, and their colonies also have a founder zooid that is different from its "daughters".
| Entoprocta | Bryozoa (Ectoprocta) | |
|---|---|---|
| Tentacles | Solid | Hollow |
| Feeding current | From bases to tips of tentacles | From tips to bases of tentacles |
| Position of 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,... |
Inside "crown" of tentacles | Outside "crown" of tentacles |
| Coelom Coelom The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size... |
none | Three-part |
| Shape of founder zooid in a colony | Same as other zooids | Round, unlike normal zooids |
| Metamorphosis to adult | Retains most larval structures | Destroys most larval structures |
| Excretory organs | Protonephridia | None |
Zooids
The body of a mature entoproct zooid has a goblet-like structure with a calyxCalyx (zoology)
-Cnidarians:The spicules containing the basal portion of the upper tentacular part of the polyp of some soft corals .-Entoprocta:A body part of the Entoprocta from which tentacles arise and the mouth and anus are located.-Echinoderms:...
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
Sphincter
A sphincter is an anatomical structure, or a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning...
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.
The stalks of colonial species arise from shared attachment plates or from a network of stolon
Stolon
In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...
s, tubes that run across a surface. In solitary species the stalk ends in a muscular sucker, or a flexible foot, or is cemented to a surface. The stalk is muscular and produces a characteristic nodding motion. In some species it is segmented
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
. Some solitary species can move, either by creeping on the muscular foot or by somersault
Somersault
A somersault is an acrobatic exercise in which a person does a full 360° flip, moving the feet over the head. A somersault can be performed either forwards, backwards, or sideways and can be executed in the air or on the ground...
ing.
The body wall consists of the epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...
and an external cuticle
Cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
, which consists mainly of criss-cross collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
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
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
.
There is no coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
(internal fluid-filled cavity lined with peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
) and the other internal organs are embedded in connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
that lies between the stomach and the base of the "crown" of tentacles. The nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
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.
Feeding, digestion, excretion, circulation and respiration
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 runs 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. 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". 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 round the groove, and then to the mouth.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 done 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
Antarctic Peninsula
The Antarctic Peninsula is the northernmost part of the mainland of Antarctica. It extends from a line between Cape Adams and a point on the mainland south of Eklund Islands....
in 1993 has cells that superficially resemble the cnidocyte
Cnidocyte
A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s of cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, 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
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...
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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and emit carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
by diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
, which works well for small animals.
Reproduction and life cycle
Most species are simultaneous hermaphroditeHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s, but some switch from males to females as they mature, while individuals of some species remain of the same sex all their lives. Individuals have one or two pairs of gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s, placed between the atrium and stomach, and opening into a single gonopore
Gonopore
A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores...
in the atrium. The eggs are thought to be fertilized in the ovaries
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...
. Most species release eggs that hatch into planktonic larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e, but a few brood their eggs in the gonopore. Those that brood small eggs nourish them by a placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
-like organ, while larvae of species with larger eggs live on stored yolk. The development of the fertilized egg into a larva follows a typical spiralian pattern: the cells divide by spiral cleavage, and mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
develops from a specific cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
labelled "4d" in the early embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. There is no coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
at any stage.
In some species the larva is a trochophore
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
which is plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic 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 genera
Genus
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...
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
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
, and the internal organs may rotate by up to 180°, so that the mouth and anus point upwards.
All species can produce clone
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
s by budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
. 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.
Classification and diversity
The phylum consists of about 150 recognized species, grouped into 4 familiesFamily (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...
:
| Family | Barentsiidae | Pedicellinidae | Loxokalypodidae | Loxosomatidae |
|---|---|---|---|---|
| Genera | Barentsia, Coriella, Pedicellinopsis, Pseudopedicellina, Urnatella | Chitaspis, Loxosomatoides, Myosoma, Pedicellina | Loxokalypus | Loxocore, Loxomitra, Loxosoma, Loxosomella, Loxosomespilon |
| Colonial | Colonial | Solitary | ||
| Septum between calyx and stalk | Yes | No | ||
| Star-cell organ | Yes | No | ||
| Anus on cone | No | Yes | ||
| Stolons present | Yes | No, colonies grow on shared baseplate | Not colonial | |
| Segmented stems | Yes | No | ||
Distribution and habitats
All species are sessile. While the great majority are marine, two species live in freshwater, Loxosomatoides sirindhornae reported in 2004 in central ThailandThailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and Urnatella gracilis found in all the continents except Antarctica. Colonial species are found in all the oceans, living on rocks, shells, algae and underwater buildings. The solitary species, which are marine, live on other animals that feed by producing water currents, such as sponges, ectoprocts and sessile annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s.
Interaction with other organisms
Some species of nudibranchNudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
s ("sea slugs") and turbellarian flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s prey on entoprocts.
Small colonies of the freshwater entoproct Urnatella gracilis have been found living on the aquatic larvae of the dobsonfly
Dobsonfly
A Dobsonfly is any insect of the subfamily Corydalinae, part of the megalopteran family Corydalidae. There are over 220 species of dobsonflies. Dobsonflies are found throughout the Americas and Asia, as well as South Africa...
Corydalus cornutus
Corydalus cornutus
The Eastern Dobsonfly, Corydalus cornutus, is a large insect in the Corydalidae family. It is found in eastern North America in regions with fast-flowing streams where its aquatic larvae develop. These are known as hellgrammites and are the top invertebrate predators in the streams in which they live...
. 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
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
, the non-colonial entoproct Loxosomella nordgaardi prefers to live attached to bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
n (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.
Entoprocts are small and have been little studied by zoologists. Hence it is hard to determine whether a specimen belongs to a species that already occurs in the same area or is an invader
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
, possibly as a result of human activities.
Fossil record
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
provided the first description of Dinomischus
Dinomischus
Dinomischus is a rare fossil animal from the Cambrian period. It reached 100 mm in height, was attached to the sea floor by a stalk, and looked loosely like a flower. The cup-shaped body at the top of the stalk probably fed by filtering the surrounding seawater, and may have created a current...
, a sessile animal with calyx, stalk and holdfast, found in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
's Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, which was formed about . Morris regarded this animal as the earliest known entoproct, since its mouth and anus lay inside a ring of structures above the calyx, but noted that these structures were flat and rather stiff, while the tentacles of modern entoprocts are flexible and have a round cross-section.
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
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
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.
Family tree
When entoprocts were discovered in the nineteenth century, they and bryozoans (ectoprocts) were regarded as classes within the phylum BryozoaBryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
, because both groups were sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animals that filter-fed
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
by means of a "crown" of tentacles that bore cilia. However, from 1869 onwards increasing awareness of differences, including the position of the entoproct 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,...
inside the feeding structure and the difference in the early pattern of division
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
of cells in their embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s, caused scientists to regard the two groups as separate phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
. "Bryozoa" then became just an alternative name for ectoprocts, in which the anus is outside the feeding organ. However studies by one team in 2007 and 2008 argues for sinking Entoprocta into Bryozoa as a class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
, and resurrecting Ectoprocta as a name for the currently identified bryozoans.
The consensus of studies from 1996 onwards has been that entoprocts are part of the Trochozoa, a protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
"superphylum" whose members are united in having as their most basic larval form the trochophore
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
type. The trochozoa also include molluscs, annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, nemertines 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.