Gill monkeys
| Gill monkeys | ||||||||||||
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Acorn worm ( Saccoglossus sp. ) |
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| Hemichordata | ||||||||||||
| Bateson , 1885 | ||||||||||||
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The hemichordate (Hemichrodata) are a phylum to which only about 100 known extant include species. They encompass two groups, which are ranked by classes in the classical system - the worm-like acorn worms (Enteropneusta) and the colony-forming, fixed (sessile) wing gills (Pterobranchia).
The gill lobsters belong to the trunk of the new mouths (Deuterostomia), to which also the chordates (chordata) and with these the vertebrates (vertebrata) belong. The scientific name Hemichordata ("half chordates") reflects the traditional view that they are the original form of the chordata. In fact, are the Hemichrodata with the echinoderms (Echinodermata) - starfish (Asteroidea), sea urchins (Echinoidea) and others - in a sister group relationship and form with them a taxon that ambulacraria is called.
anatomy
Gill insects have a soft, worm-like, but (apart from a rough three-way division) internally unsegmented body, which, as with all bilaterally symmetrical animals, has a mirror symmetry and thus has three clearly defined body axes. They come in different colors from white to dark purple and can be up to 2.50 m long, but on the other hand there are also only millimeter-sized animals. The division of the body into three parts is characteristic: the foremost section is the head lobe ( prosoma ), which is shield-shaped in the class of wing gills (pterobranchia) and is therefore also known as the head or rostral shield. This is followed by a short collar ( mesosoma ), in which the mouth opening is accommodated, and a long trunk ( metasoma ). This basic tripartite division of the body also affects the body cavity, the coelom , which is divided into pro , meso and metacoel , which are spatially located in the corresponding body sections.
Especially in the class of acorn worms (Enteropneusta) there are up to a hundred gill slits , which give the group its name , through which the animals breathe and through which the water sucked in through the mouth can flow out again while retaining the food particles. They connect the first section of the intestine, the gill intestine , with the outside world. The wing gills, on the other hand, either have only two paired gills or they are completely absent.
As a bulge of the stomach in the mouth lobe (buccal pocket) there is another characteristic structure of the acorn worms, namely the stomochord . It used to be considered a possible forerunner of the notochord , i.e. the elastic, rod-shaped support structure of the chordates . However, since it has no essential similarities with this and is never used as a hydrostatic skeleton , this view is hardly ever scientifically supported today.
The open blood vessel system of the gill lobsters is quite primitive, only in the collar and head flap there are two real blood channels. The very elementary "heart" is also located in the head flap and consists of a single contractile blood vessel that pumps blood arriving on the back (dorsal) into the ventral (ventral) vein. The blood flow is therefore directed (unidirectional).
Liquid waste is mainly excreted through the skin; there is also a membrane system called a glomerulus in which blood flowing in from the heart is filtered several times. The resulting urine is released into the body cavity of the head flap and from there passes through a pore to the outside.
The nervous system essentially consists of an abdominal and a rear nerve cord, which are connected in a ring in the head flap and around the intestine and send nerve endings into the outer skin (epidermis). In acorn worms, the back nerve runs in a special fold in the collar. This hollow nerve cord, the "collar marrow" of the acorn worms, is sometimes viewed as a homologue of the spinal cord of the chordates , primarily because of its embryonic origin , so both would have arisen from the same predecessor structure.
Diet, Habitat and Distribution
Gill monkeys can feed in two different ways: Either they burrow through the sediment of the sea floor, i.e. they ingest soil sludge and digest the organic content it contains, like an earthworm , or they filter food particles floating freely in the water, such as the Example algae . They therefore mostly live in or below the intertidal zone , on or in the bottom of the sea ( benthic ), sometimes down to depths of 5000 meters, and often form U-shaped caves there. Only a few species live in the open sea ( pelagic ). Gill insects are distributed worldwide in all marine waters from the tropics to polar zones.
Reproduction
Gill insects have separate sexes, which, however, hardly differ externally. From the fertilized eggs mostly ciliated larvae (" tornaria - larvae ") develop , which resemble the larvae of echinoderms and from which the adult animals develop. They spend part of their life cycle before metamorphosis in the plankton , where they feed on fine food particles that get caught in the larvae 's eyelash bands and are transported from there to the mouth; they are therefore also called planktotroph . In some species, however, the development also happens directly, i.e. without an intervening larval stage.
In addition to sexual reproduction, there is also asexual reproduction: a young animal simply strangles itself off from the genetically identical parent animal in a process called budding . In addition, it can also happen that an individual simply splits into two parts, which then continue to exist independently of one another.
Fossils
The first fossil finds of gill insects come from the Chinese Chengjiang faunal community and the somewhat more recent Canadian Burgess slate , which originated in the geological age of the Cambrian . A now extinct group of gill animals, the graptolites (Graptolithina), even form important key fossils for the Ordovician and Silurian .
Systematics
A distinction is made between two classes of gill lobsters still alive today.
- The acorn worms (Enteropneusta) comprise about 90 to 100 species in four families . They live as solitary animals and, in contrast to the wing gills, are not equipped with tentacles. Their size varies between 2.5 and 250 centimeters.
- The approximately twenty species of the wing gill (Pterobranchia) have a more vase-shaped body, have tentacles and are no more than one centimeter long. Unlike acorn worms, they live in colonies in which the individual animals (zooids) are connected to one another by tubular runners, namely stolons. Such a colony is often surrounded by a network of cavities made of collagen .
The assignment of the species Planctosphaera pelagica , for which a separate class Planctosphaeroidea is sometimes distinguished, is uncertain and puzzling . Of this species, however, only the characteristic rounded tornaria larva is known, which lives in the open ocean and can grow up to one centimeter in size. Most researchers assume that it belongs to a previously unknown species of acorn worm that is believed to live in the deep sea.
The graptolites (Graptolithina) also form an important group of extinct gill animals; their fossils look like little saw blades wound in a spiral. It is believed that this is an adaptation to floating in free ocean water (their pelagic way of life). After the discovery of the wing-gill species Cephalodiscus graptolitoides off the coast of New Caledonia, which in its physique shows astonishing similarity to the fossil graptolites, most researchers assume that the graptolites also belonged to the wing-gill species.
Tribal history
After some doubts regarding the monophyly of the gill lobsters and their subgroups, the monophyly of all three taxa is today the most probable hypothesis according to morphological and genetic data, even if the genetic data leave certain doubts open as to whether the wing gills should not be positioned within the acorn worms (which would make this paraphyletic in this case).
The following cladogram shows the most likely relationships based on current knowledge:
| Neumünder (Deuterostomia) |
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literature
- DT Anderson: Invertebrate Zoology. Cape. 17. Oxford Univ. Press, Oxford 2001 (2nd ed.), P. 418, ISBN 0-19-551368-1
- Richard SK Barnes, P. Calow, PJW Olive, DW Golding, JI Spicer: The invertebrates - a synthesis. Cape. 7.2. 3rd ed., Blackwell, Oxford 2001, p. 147, ISBN 0-632-04761-5
- Richard C. Brusca, GJ Brusca: Invertebrates. Cape. 23. Sinauer, Sunderland Mass 2003 (2nd ed.), P. 847, ISBN 0-87893-097-3
- A. Goldschmid: Hemichordata (Branchiotremata). in: Wilfried Westheide: Special Zoology. Part 1. Protozoa and invertebrates. Gustav Fischer, Stuttgart- Jena 1996, Akademie Verlag, Heidelberg 2004, ISBN 3-8274-1482-2
- EE Ruppert, RS Fox, RP Barnes: Invertebrate Zoology - A functional evolutionary approach. Cape. 27. Brooks / Cole 2004, p. 857, ISBN 0-03-025982-7
- J. van der Land: Hemichordata. in: MJ Costello u. a. (Ed.): European register of marine species - a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels. Vol 50. Publication scientifiques du MNHN, Paris 2001, pp. 336f, ISBN 2-85653-538-0
Individual evidence
- ↑ CB Cameron (2005): A phylogeny of the hemichordates based on morphological characters. Canadian Journal of Zoology 83: 196-215. doi : 10.1139 / Z04-190
- ↑ Oleg Simakov et al. (2015): Hemichordate genomes and deuterostome origins. Nature 527: 459-465. doi : 10.1038 / nature16150
- ^ Johanna T. Cannon, Amanda L. Rychel, Heather Eccleston, Kenneth M. Halanych, Billie J. Swalla (2009): Molecular phylogeny of hemichordata, with updated status of deep-sea enteropneusts. Molecular Phylogenetics and Evolution 52: 17-24. doi : 10.1016 / j.ympev.2009.03.027
- ↑ Casey W. Dunn, Gonzalo Giribet, Gregory D. Edgecombe, Andreas Hejnol (2014): Animal Phylogeny and Its Evolutionary Implications. Annual Review of Ecology, Evolution, and Systematics 45: 371-395. doi : 10.1146 / annurev-ecolsys-120213-091627
- ↑ Maximilian J. Telford, Graham E. Budd, Hervé Philippe (2015): Phylogenomic Insights into Animal Evolution. Current Biology 25: R876-R887. doi : 10.1016 / j.cub.2015.07.060
