Misplaced Pages

#425574

81-411: Nemertini Nemertinea Rhynchocoela Nemertea is a phylum of animals also known as ribbon worms or proboscis worms , consisting of about 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below

162-1269: A clade separate from the Hoplonemertea , while Thollesson & Norenburg (2003) believe the Bdellonemertea are a part of the Monostilifera (with one active stylet), which are within the Hoplonemertea – which implies that "Enopla" and "Hoplonemertea" are synonyms for the same branch of the tree. The Polystilifera (with many tiny stylets) are monophyletic. English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre- cladistics analyses emphasised as shared features: multiciliated (with multiple cilia per cell), glandular epidermis; rod-shaped secretory bodies or rhabdites; frontal glands or organs; protonephridia ; and acoelomate body organization. However, multiciliated epidermal cells and epidermal gland cells are also found in Ctenophora , Echiura , Sipuncula , Annelida , Mollusca and other taxa . The rhabdites of nemertea have

243-466: A convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned,

324-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in

405-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make

486-422: A different structure from those of flatworms at the microscopic scale. The frontal glands or organs of flatworms vary a lot in structure, and similar structures appear in small marine annelids and entoproct larvae. The protonephridia of nemertea and flatworms are different in structure, and in position – the flame cells of nemertea are usually in the walls of the fluid vessels and are served by "drains" from which

567-553: A dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places. Most nemerteans are carnivores , feeding on annelids , clams and crustaceans . Some species of nemerteans are scavengers , and a few live commensally inside the mantle cavity of molluscs . In most species the sexes are separate, but all the freshwater species are hermaphroditic . Nemerteans often have numerous temporary gonads ( ovaries or testes ), and build temporary gonoducts (ducts from which

648-447: A few types of nemerteans, and is assumed to be a derived feature. All reproduce sexually , and most species are gonochoric (the sexes are separate), but all the freshwater forms are hermaphroditic . Nemerteans often have numerous temporary gonads ( ovaries or testes ), forming a row down each side of the body in the mesenchyme . Temporary gonoducts (ducts from which the ova or sperm are emitted), one per gonad, are built when

729-399: A fluid skeleton in everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation. The circulatory vessels are a system of coeloms. In the simplest type of circulatory system, two lateral vessels are joined at the ends to form a loop. However, many species have additional long-wise and cross-wise vessels. There

810-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,

891-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since

SECTION 10

#1732775631426

972-554: A lot, up to 10 times their resting length in some species, but reduce their length to 50% and increase their width to 300% when disturbed. A few have relatively short but wide bodies, for example Malacobdella grossa is up to 3.5 centimetres (1.4 in) long and 1 centimetre (0.39 in) wide, and some of these are much less stretchy. Smaller nemerteans are approximately cylindrical, but larger species are flattened dorso-ventrally . Many have visible patterns in various combinations of yellow, orange, red and green. The outermost layer of

1053-495: A marine worm which was apparently 17.76 metres (58.3 ft) long ("40 cubits"), about the width of a child's arm, and whose touch made a hand swell. William Borlase wrote in 1758 of a "sea long worm", and in 1770 Gunnerus wrote a formal description of this animal, which he called Ascaris longissima . Its current name, Lineus longissimus , was first used in 1806 by Sowerby. In 1995, a total of 1,149 species had been described and grouped into 250 genera. Nemertea are named after

1134-438: A number of pigment-cup ocelli , which can detect light but can not form an image. Nemerteans respire through the skin. They have at least two lateral vessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid. There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in

1215-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by

1296-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae

1377-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to

1458-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on

1539-480: A specimen of Lineus longissimus , Ruppert, Fox and Barnes refer to a Lineus longissimus 54 metres (177 ft) long, washed ashore after a storm off St Andrews in Scotland. Other estimates are about 30 metres (98 ft). Zoologists find it extremely difficult to measure this species. For comparison: L. longissimus , however, is usually only a few millimeters wide. The bodies of most nemerteans can stretch

1620-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display

1701-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with

SECTION 20

#1732775631426

1782-482: Is monophyletic , its main synapomorphies being the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related to flatworms , but both phyla are regarded as members of the Lophotrochozoa , a very large clade, sometimes viewed as a superphylum that also includes molluscs , annelids , brachiopods , bryozoa and many other protostomes . In 1555 Olaus Magnus wrote of

1863-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by

1944-484: Is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and

2025-700: Is a native of Australia but has been found in the British Isles , in Sao Miguel in the Azores , in Gran Canaria , and in a lava tube at Kaumana on the Island of Hawaii . It can build a cocoon, which allows it to avoid desiccation while being transported, and it may be able to build populations quickly in new areas as it is a protandrous hermaphrodite . Another terrestrial genus , Geonemertes ,

2106-418: Is an infolding of the body wall, and sits in the rhynchocoel when inactive. When muscles in the wall of the rhynchocoel compress the fluid inside, the pressure makes the proboscis jump inside-out along a canal called the rhynchodeum and through an orifice, the proboscis pore. The proboscis has a muscle which attaches to the back of the rhynchocoel, can stretch up to 30 times its inactive length and acts to retract

2187-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,

2268-501: Is derived from cells labelled 3a and 3b, and endomesoderm (inner part of the mesoderm) is derived from the 4d cell. Some of the ectomesoderm in annelids , echiurans and molluscs is derived from cells 3a and 3b, while the ectomesoderm of polyclad flatworms is derived from the 2b cell and acoel flatworms produce no ectomesoderm. In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellular connective tissue . This structure

2349-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish

2430-1165: Is mostly found in Australasia but has species in the Seychelles , widely across the Indo-Pacific , in Tristan da Cunha in the South Atlantic, in Frankfurt , in the Canary Islands , in Madeira and in the Azores. Geonemertes pelaensis has been implicated in the decline of native arthropod species on the Ogasawara Islands , where it was introduced in the 1980s. Most are carnivores , feeding on annelids , clams and crustaceans , and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects and myriapods are

2511-566: Is no doubt that the phylum Nemertea is monophyletic (meaning that the phylum includes all and only descendants of one ancestor that was also a member of the phylum). The synapomorphies (trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel. While Ruppert, Fox & Barnes (2004a) treat the Palaeonemertea as monophyletic, Thollesson & Norenburg (2003) regard them as paraphyletic and basal (contains

Nemertea - Misplaced Pages Continue

2592-403: Is no heart nor pumping vessels, and the flow of fluid depends on contraction of both the vessels and the body wall's muscles. In some species, circulation is intermittent, and fluid ebbs and flows in the long-wise vessels. The fluid in the vessels is usually colorless, but in some species it contains cells that are yellow, orange, green or red. The red type contain hemoglobin and carry oxygen, but

2673-409: Is segmented. But this is assumed to be a derived trait. The segmentation does not include the coelom and body wall, and is therefore referred to as pseudosegmentation. The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and the anus is at the tip of the tail. Above the gut and separated from the gut by mesenchyme

2754-478: Is similar to that found in larger flatworms such as polyclads and triclads , but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among the Spiralia (animals in which the early cell divisions make a spiral pattern) such as sipunculans , echiurans and many annelids. Acoela Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla )

2835-410: Is the rhynchocoel , a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class Anopla has an orifice a little to the front of the mouth, but still under the front of the body. In the other class, Enopla , the mouth and the front of the rhynchocoel share an orifice. The rhynchocoel is a coelom , as it is lined by epithelium . The proboscis

2916-521: Is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide

2997-620: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Class (biology) The class as a distinct rank of biological classification having its own distinctive name – and not just called a top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in

3078-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to

3159-465: The Pennsylvanian -age animals from Mazon Creek in northern and central Illinois , may be a nemertean. This fossil, however, only preserves the outline of the "worm", and there is no evidence of a proboscis, so there is no certainty that it represents a nemertean.   Palaeonemertea   Heteronemertea     Bdellonemertea   Monostilifera Polystilifera There

3240-411: The holdfasts of algae or sessile animals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined with mucus or produce cellophane -like tubes. Mainly in the tropics and subtropics, about 12 species appear in freshwater, and about a dozen species live on land in cool, damp places, for example under rotting logs. The terrestrial Argonemertes dendyi

3321-416: The suborder Monostilifera , whose proboscis have one active stylet, move by extending the proboscis, sticking it to an object and pulling the animal toward the object. Larger species often break up when stimulated, and the fragments often grow into full individuals. Some species fragment routinely and even parts near the tail can grow full bodies. But this kind of extreme regeneration is restricted to only

Nemertea - Misplaced Pages Continue

3402-535: The Burgess Shale fossils show no evidence of rhynchocoel nor intestinal caeca. Knaust & Desrochers (2019) reported fossils of vermiform organisms with a wide range of morphologies occurring on bedding planes from the Late Ordovician ( Katian ) Vauréal Formation ( Canada ). In the specimens preserving the anterior end of the body, this end is pointed or rounded, bearing a rhynchocoel with

3483-646: The Greek sea- nymph Nemertes, one of the daughters of Nereus and Doris . Alternative names for the phylum have included Nemertini , Nemertinea , and Rhynchocoela . The Nemertodermatida are a separate phylum, whose closest relatives appear to be the Acoela . The typical nemertean body is very thin in proportion to its length. The smallest are a few millimeters long, most are less than 20 centimetres (7.9 in), and several exceed 1 metre (3.3 ft). The longest animal ever found, at 54 metres (177 ft) long, may be

3564-534: The South African Polybrachiorhynchus dayi , both called "tapeworms" in their respective localities, are sold as fish bait. Traditional taxonomic classification has divided the group into two classes and four orders: Recent molecular phylogenetic studies divided the group into two superclasses, three classes, and eight orders: As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare. One might expect

3645-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista

3726-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine

3807-658: The ancestors of the more recent clades). The Anopla ("unarmed") represent an evolutionary grade of nemerteans without stylets (comprising the Heteronemertea and the Palaeonemerteans), while Enopla ("armed") are monophyletic, but find that Palaeonemertea is doubly paraphyletic, having given rise to both the Heteronemertea and the Enopla. Ruppert, Fox & Barnes (2004a) treat the Bdellonemertea as

3888-447: The animal experiences anoxia , for example while burrowing in oxygen-free sediments . Some species have paired cerebral organs, sacs whose only openings are to the outside. Others species have unpaired evertible organs on the front of their heads. Some have slits along the side of the head or grooves obliquely across the head, and these may be associated with paired cerebral organs. All of these are thought to be chemoreceptors , and

3969-487: The back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host . The brain is a ring of four ganglia , positioned around the rhynchocoel near the animal's front end. At least a pair of ventral nerve cords connect to the brain and run along the length of the body. Most nemerteans have various chemoreceptors , and on their heads some species have

4050-425: The body has no cuticle , but consists of a ciliated and glandular epithelium containing rhabdites , which form the mucus in which the cilia glide. Each ciliated cell has many cilia and microvilli . The outermost layer rests on a thickened basement membrane , the dermis . Next to the dermis are at least three layers of muscles, some circular and some longitudinal. The combinations of muscle types vary between

4131-446: The cerebral organs may also aiding osmoregulation . Small pits in the epidermis appear to be sensors. On their head, some species have a number of pigment-cup ocelli , which can detect light but not form an image. Most nemerteans have two to six ocelli, although some have hundreds. A few tiny species that live between grains of sand have statocysts , which sense balance. Paranemertes peregrina , which feeds on polychaetes, can follow

SECTION 50

#1732775631426

4212-411: The class Enopla , the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has a stylet , a calcareous barb, with which the animal stabs the prey many times to inject toxins and digestive secretions. The prey is then swallowed whole or, after partial digestion, its tissues are sucked into the mouth. The stylet is attached about one-third of the distance from

4293-420: The classification of plants that appeared in his Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with a distinct type of construction, which

4374-417: The different classes , but these are not associated with differences in movement. Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species. Inside the concentric tubes of these layers is mesenchyme , a kind of connective tissue . In pelagic species this tissue is gelatinous and buoyant. They are unsegmented, but at least one species, Annulonemertes minusculus,

4455-508: The end of the everted proboscis, which extends only enough to expose the stylet. On either side of the active stylet are sacs containing back-up stylets to replace the active one as the animal grows or an active one is lost. Instead of one stylet, the Polystilifera have a pad that bears many tiny stylets, and these animals have separate orifices for the proboscis and mouth, unlike other Enopla. The Enopla can only attack after contacting

4536-461: The fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or to form planuliform larvae . The planuliform larva stage may be short-lived and lecithotrophic ("yolky") before becoming a juvenile, or may be planktotrophic , swimming for some time and eating prey larger than microscopic particles. However, many members of

4617-459: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia

4698-404: The front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside. All nemerteans move slowly, using their external cilia to glide on surfaces on a trail of slime , while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations. A few live in the open ocean while the rest find or make hiding places on the bottom. About

4779-468: The function of the other pigments is unknown. Nemertea use organs called protonephridia to excrete soluble waste products, especially nitrogenous by-products of cellular metabolism . In nemertean protonephridia, flame cells which filter out the wastes are embedded in the front part of the two lateral fluid vessels. The flame cells remove the wastes into two collecting ducts, one on either side, and each duct has one or more nephridiopores through which

4860-531: The fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as

4941-508: The juvenile. When it is fully formed, the juvenile bursts out of the larva body and usually eats it during this catastrophic metamorphosis . This larval stage is unique in that there are no Hox genes involved during development, which are only found in the juveniles developing inside the larvae. The species Paranemertes peregrina has been reported as having a life span of around 18 months. Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or

SECTION 60

#1732775631426

5022-427: The midline of the body, the anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel , a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in

5103-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within

5184-426: The only known prey of the two terrestrial species of Argonemertes . A few nemerteans are scavengers , and these generally have good distance chemoreception ("smell") and are not selective about their prey. A few species live commensally inside the mantle cavity of molluscs and feed on micro-organisms filtered out by the host. Near San Francisco the nemertean Carcinonemertes errans has consumed about 55% of

5265-427: The order Heteronemertea and the palaeonemertean family Hubrechtiidae form a pilidium larva, which can capture unicellular algae and which Maslakova describes as like a deerstalker cap with the ear flaps pulled down. It has a gut which lies across the body, a mouth between the "ear flaps", but no anus. A small number of imaginal discs form, encircling the archenteron (developing gut) and coalesce to form

5346-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of

5427-470: The ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, some lay them in a burrow or tube, and some protect them by cocoons or gelatinous strings. Some bathypelagic (deep sea) species have internal fertilization , and some of these are viviparous , growing their embryos in the female's body. The zygote (fertilised egg) divides by spiral cleavage and grows by determinate development , in which

5508-401: The ova or sperm are emitted) opening to a gonopore , one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. The fertilized egg divides by spiral cleavage and grows by determinate development , in which the fate of a cell can usually be predicted from its predecessors in

5589-556: The prey's trails of mucus, and find its burrow by backtracking along its own trail of mucus. Nemerteans generally move slowly, though they have occasionally been documented to successfully prey on spiders or insects. Most nemerteans use their external cilia to glide on surfaces on a trail of slime , some of which is produced by glands in the head. Larger species use muscular waves to crawl, and some aquatic species swim by dorso-ventral undulations. Some species burrow by means of muscular peristalsis , and have powerful muscles. Some species of

5670-402: The prey. Some nemerteans, such as L. longissimus , absorb organic food in solution through their skins, which may make the long, slim bodies an advantage. Suspension feeding is found only among the specialized symbiotic bdellonemerteans , which have a proboscis but no stylet, and use suckers to attach themselves to bivalves . Nemerteans lack specialized gills , and respiration occurs over

5751-400: The proboscis, which is characteristic for nemerteans. The authors attributed these fossils to nemerteans and interpreted them as the oldest record of the group reported so far. However, Knaust & Desrochers cautioned that partly preserved putative nemertean fossils might ultimately turn out to be fossils of turbellarians or annelids . It has been suggested that Archisymplectes , one of

5832-415: The proboscis. The proboscis of the class Anopla exits from an orifice which is separate from the mouth, coils around the prey and immobilizes it by sticky, toxic secretions. The Anopla can attack as soon as the prey moves into the range of the proboscis. Some Anopla have branched proboscises which can be described as "a mass of sticky spaghetti". The animal then draws its prey into its mouth. In most of

5913-430: The process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or larvae that resemble the planulas of cnidarians . However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some species fragment readily, and even parts cut off near

5994-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On

6075-484: The rhynchocoel near its front end – while the brains of most protostome invertebrates encircle the foregut. Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord. In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers. The central nervous-system is often red or pink because it contains hemoglobin . This stores oxygen for peak activity or when

6156-572: The stylet of a nemertean to be preserved, since it is made of calcium phosphate , but no fossil stylets have yet been found. Knaust (2010) reported nemertean fossils and traces from the Middle Triassic of Germany . The Middle Cambrian fossil Amiskwia from the Burgess Shale has been classed as a nemertean, based on a resemblance to some unusual deep-sea swimming nemerteans, but few paleontologists accept this classification as

6237-422: The surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid circulation rather than diffusion to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels, while their blood is contained in the main body cavity. The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as

6318-418: The tail can grow full bodies. Traditional taxonomy divides the phylum in two classes , Anopla ("unarmed" – their proboscises do not have a little dagger) with two orders , and Enopla ("armed" with a dagger) also with two orders. However, it is now accepted that Anopla are paraphyletic , as one order of Anopla is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea itself

6399-553: The total egg production of its host, the dungeness crab Metacarcinus magister . C. errans is considered a significant factor in the collapse of the dungeness crab fishery. Other coastal nemerteans have devastated clam beds. The few predators on nemerteans include bottom-feeding fish, some sea birds, a few invertebrates including horseshoe crabs , and other nemerteans. Nemerteans' skins secrete toxins that deter many predators, but some crabs may clean nemerteans with one claw before eating them. The American Cerebratulus lacteus and

6480-405: The wastes exit by a small number of tubes through the skin, while the flame cells of flatworms are scattered throughout the body. Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia. According to more recent analyses, in the development of nemertean embryos, ecto mesoderm (outer part of the mesoderm, which is the layer in which most of the internal organs are built)

6561-439: The wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be that osmoregulation is more difficult in non-marine environments. The central nervous-system consists of a brain and paired ventral nerve cords that connect to the brain and run along the length of the body. The brain is a ring of four ganglia , masses of nerve cells, positioned round

#425574