Acanthocephala. The Acanthocephala were thought to be a discrete phylum.
Recent genome analysis has shown that they are descended from, and should be considered as, highly modified rotifers. This is an example of molecular phylogenetics. This unified taxon is known as Syndermata. History The earliest recognisable description of Acanthocephala – a worm with a proboscis armed with hooks – was made by Italian author Francesco Redi (1684). In 1771, Joseph Koelreuter proposed the name Acanthocephala. Philipp Ludwig Statius Müller independently called them Echinorhynchus in 1776. Karl Rudolphi in 1809 formally named them Acanthocephala.
Phylogenetic relationships Acanthocephalans are highly adapted to a parasitic mode of life, and have lost many organs and structures through evolutionary processes. The three rotifer classes and the Acanthocephala make up a clade called Syndermata. This clade is placed in the Platyzoa. Morphological characteristics Acoelomorpha. Earlier (2007) work dismissed the phylum as paraphyletic, with Acoela and Nemertodermatida as separate clades. An ongoing (Feb. 2011) collaborative research project has "the researchers ... confident that they can reach an agreement about where acoels fit in evolutionary history". Anatomy They are simultaneous hermaphrodites, but have no gonads and no ducts associated with the female reproductive system.
Instead, gametes are produced from the mesenchymal cells that fill the body between the epidermis and the digestive vacuole. References Jump up ^ Nielsen, C. (2012). Annelid. Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied.
The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvae, which live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites and produce a ring-like cocoon round their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.
Earthworms support terrestrial food chains both as prey and by aerating and enriching soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Arthropod. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Brachiopod. Lifespans range from three to over thirty years.
Ripe gametes (ova or sperm) float from the gonads into the main coelom and then exit into the mantle cavity. Bryozoa. The phylum was originally called "Polyzoa", but this term was superseded by "Bryozoa" in 1831.
Another group of animals discovered subsequently, whose filtering mechanism looked similar, was also included in "Bryozoa" until 1869, when the two groups were noted to be very different internally. The more recently discovered group was given the name Entoprocta, while the original "Bryozoa" were called "Ectoprocta". Chaetognatha. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large. Anatomy Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle.
Chordate. This article is about the animal phylum.
For the leaf shape, see Cordate. The phylum Hemichordata including the acorn worms has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the phylum Echinodermata, including starfish, sea urchins, and sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion. Cnidaria.
Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from endosymbiotic algae, and a few are parasites.
Many are preyed upon by other animals including starfish, sea slugs, fish and turtles. Coral reefs, whose polyps are rich in endosymbiotic algae, support some of the world's most productive ecosystems, and protect vegetation in tidal zones and on shorelines from strong currents and tides. While corals are almost entirely restricted to warm, shallow marine waters, other cnidarians live in the depths, in polar seas and in freshwater. Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. Fossils of cnidarians that do not build mineralized structures are very rare.
Distinguishing features Ctenophora. Almost all ctenophores are predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed.
In favorable circumstances, ctenophores can eat ten times their own weight in a day. Only 100–150 species have been validated, and possibly another 25 have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with colloblasts, sticky cells that capture prey. Dicyemida. Echinoderm. Taxonomy and evolution Along with the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the animal kingdom, the other being the protostomes. Entoprocta. 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 larvae 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 stolons. Nudibranchs ("sea slugs") of the genus Trapania and turbellarian flatworms prey on entoprocts. A few entoproct species have been found living in close association with other animals. Fossils of entoprocts are very rare, and the earliest specimens that have been identified with confidence date from the Late Jurassic.
Flatworm. In traditional zoology texts, Platyhelminthes are divided into Turbellaria, which are mostly nonparasitic animals such as planarians, and three entirely parasitic groups: Cestoda, Trematoda and Monogenea; however, since the turbellarians have since been proven not to be monophyletic, this classification is now deprecated. Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments such as leaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts.
Gastrotrich. Etymology and taxonomy The name "gastrotrich" comes from the Greek γαστήρ gaster, meaning "stomach", and θρίξ thrix, meaning "hair". The name was coined by the Russian zoologist Élie Metchnikoff in 1865. The common name "hairyback" apparently arises from a mistranslation of "gastrotrich". Anatomy In the chaetonotidans, the excretory system consists of a single pair of protonephridia, which open through separate pores on the lateral underside of the animal, usually in the midsection of the body.
In the macrodasyidans, there are several pairs of these opening along the side of the body. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation. Unusually, the protonephridia do not take the form of flame cells, but, instead, the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum.
Gnathostomulid. Hemichordate. Acorn worms are solitary worm-shaped organisms. Kinorhyncha. Limnognathia. L. maerski has very complicated jaws, with fifteen separate elements. Loricifera. Mollusca. A striking feature of molluscs is the use of the same organ for multiple functions. Nematode. Nematomorpha. Description and biology Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. Nemertea. Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although most are less than 20 centimetres (7.9 in) long, one specimen has been estimated at 54 metres (177 ft), which would make it the longest animal ever found. Most are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies.
Onychophora. Orthonectida. Phoronid. Placozoa. Priapulida. Rotifer. Sipuncula. Sponge. Symbion. Tardigrade. Xenoturbella.