Acoelomate Bilateral Animals - Unit 4 PDF

Acoelomate bilateral animals The phyla Platyhelminthes, Nemertea and Gnathostomulida together form the superphylum Lophotrochozoa, and are the simplest animals to have bilateral symmetry. All are loosely considered 'worms', as they are all elongated, bilateral invertebrate animals without appendages...

Acoelomate bilateral animals The phyla Platyhelminthes, Nemertea and Gnathostomulida together form the superphylum Lophotrochozoa, and are the simplest animals to have bilateral symmetry. All are loosely considered 'worms', as they are all elongated, bilateral invertebrate animals without appendages. They all have a single internal cavity (used in digestion). The area between the ectoderm and endoderm is filled with mesoderm tissues (muscle cells, etc.). Thus, as they have three well-defined layers, they are termed triploblastic. As they lack a coelom or a pseudocoelom, they are termed acoelomate, and show a higher level of organisation than the radiate animals discussed in the previous module. The mesoderm makes the formation of more elaborate organs possible, and they are said to have reached the 'organ-system' level of organisation. This group is important zoologically, as they are the most primitive forms to have developed bilateral symmetry, and are the beginning of cephalisation, a feature, which defines the vast majority of higher animals. Some centralisation of the nervous system occurs, and this is the first group of animals in which an excretory system occurs. Nemerteans are the simplest animals to have a circulatory system with blood, and a one-way alimentary canal. Phylum Platyhelminthes Flatworms range in size from less than a millimetre, to some tapeworms, which are many metres in length. They have flattened bodies, which may be slender, broadly leaf-like, or long and ribbon-like. With about 25,000 known species, they are the largest phylum of acoelomate. Flatworms are found in marine environments, freshwater, and even damp terrestrial environments. A terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which has rapidly colonised large areas of Ireland and Scotland since its unintentional introduction in the 1960s, and has since destroyed most of the indigenous earthworms. Most flatworms are free-living, but many are parasitic. There are four classes: Trematoda (flukes), Cestoda (tapeworms), Monogenea and Turbellaria. The Turbellaria are mostly free-living worms, ranging from less than 5mm to 50cm. They are typically creeping forms that combine cilliary action with muscular movement. Smaller species rely more on cilliary action, larger ones more on muscular activity. Mouths are ventral, and they have the simplest life cycles of the phylum. Active locomotion has favoured cephalisation in the nervous system, and the development of sense organs. Light sensitive eyespots (Ocelli) are common in Tubellarians, and larval trematodes. Tactile and chemoreceptor cells are abundant over the body and the planarian group form definite organs on the auricles (earlike lobes on the sides of the head). Many Tubellarians reproduce both asexually (by fission) and sexually. Tubellarians have considerable powers of regeneration; a piece of tissue excised from the centre of an individual can grow a head and 'tail' end, forming a new individual. Like the other members of the phylum, they are hermaphroditic, but exchange gametes with another individual. Three common marine flatworms Tubellarians anatomy A: Paraplanocera oligoglena. B: Pseudoceros sp. C: Eurylepta fuscopunctatus. The Trematoda are estimated to include 18,000 to 24,000 species. Nearly all trematodes are parasites of molluscs (intermediate host) and vertebrates (final host). Bodies of adults are leaf like or cylindrical in shape and do not have cilia. They usually have ventral suckers, and their alimentary canal consists of two main branches. Trematodes can also be classified into two groups, according to the system, which they infect in the host. Tissue flukes infect the bile ducts, lungs, or other biological tissues. This group includes the lung fluke, Paragonimus westermani, and the liver flukes, Clonorchis sinensis and Fasciola hepatica. Blood flukes inhabit the blood in some stages of their life cycle, e.g. species of the genus Schistosoma. All trematodes infect molluscs and most have a complex life cycle involving other hosts. Most are monoecious and alternately reproduce sexually and asexually. In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host faeces. Eggs shed in water release free-swimming larval forms that are infective to the intermediate host, in which asexual reproduction occurs. Several species directly infect humans, including the liver fluke Clonorchis, which is common in many areas of Asia, particularly China, South East Asia and Japan. Cats, dogs and pigs are also infected. Its intermediate hosts are snails and fish. Humans can become infected by eating infected fish. Individual flukes may live for 15-30 years in the bile ducts of infected humans. The blood fluke causes one of the most important infectious diseases worldwide, with 200 million people infected. The disease is common over much of Africa, South America, West Indies, Middle East, and Far East. Unfortunately, schemes intended to raise the standard of living in some developing countries have increased the prevalence of the disease, e.g. the construction of the Aswan Dam in Egypt created more habitats for the intermediate snail hosts. Before the Dam was built, annual flooding and drying out killed many snails. Four years post construction, infection in fishermen in the area rose from a very low level to 76%. The Cestoda have long flat bodies in which there is a lineal set of reproductive organs, and they attach to the hosts intestine wall via the head or scolex. They are all parasitic. The adults absorb food pre-digested by the host, so the worms have no need for a digestive tract or a mouth. With rare exceptions, all cestodes require at least 2 intermediate hosts. Tapeworms can grow 15ft to 30ft (10m) in length. Symptoms in infective humans may include upper abdominal discomfort, diarrhoea, and loss of appetite. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and, if left untreated, can cause intestinal blockages. However, infestations are usually asymptomatic. Most enter the primary host through infected food. Two important Cestodes are the pork tapeworm, Taenia solium, and the beef tapeworms, T. saginata. Cysticercosis, which is a dangerous complication of Taenia solium, may occur when the larvae develop outside the intestinal tract. This parasite can move from the intestines to muscle tissue, bone marrow, fingers, and, in some cases, the central nervous system, leading to seizures and other neurological problems. Adult tapeworms share a basic body structure. All have a scolex, sometimes colloquially referred to as the 'head,' a 'neck,' and one or more proglottids, which are sometimes called 'segments.' These are the source of the name 'tapeworm,' because they look like a strip of tape. All cestodes have a nerve ring in the scolex with lateral trunks passing through the rest of the body. The Scolex or 'head' of the animal attaches to the intestine of the definitive host, by sucker like organs, often with hooks or spiny tentacles. The neck area is a relatively undifferentiated mass of cells that divide to form new proglottid 'segments.' This is where all growth in an adult tapeworm occurs. The body is composed of successive units posterior to the scolex, the proglottids. Each proglottid has areas of muscle weakness at the anterior and posterior ends. The sum of the proglottids is called a strobila. Like some other flatworms, cestodes use flame cells (protonephridia) for excretion, which are located in proglottids. Mature or gravid proglottids are released from the mature tapeworm and leave the host in its faeces. As each proglottid contains both male and female reproductive structures, they can reproduce independently. It has been suggested by some biologists that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids. Phylum Nemertea Individuals of this group (ribbon worms) are bilaterally symmetrical, and have highly contractile bodies, which are cylindrical interiorly, and flattened posteriorly. They have three germ layers, and an epidermis with cilia and gland cells. A distinguishing feature from the platyhelminthes is the eversible (extendable) proboscis, and a complete digestive tract form mouth to anus. The body wall has two layers of muscle (outer circular, inner longitudinal). The nervous system consists of usually a four lobed brain, and sensory ciliated pits are found on either side of the head. An excretory system with flame cells is present, but there is no respiratory system. The sexes are separate, with simple gonads, but asexual reproduction via fission also takes place. Nemerteans are carnivorous, feeding primarily on annelids and other small invertebrates. Prey is seized with the proboscis (a large muscular tube with an anterior barb), which is sticky and coiled around the prey, whilst pouring a toxin onto it. Phylum Gnathostomulida These are curious delicate worm-like animals 0.5mm-1mm long. There have been 80 species described since 1956, when the first published description occurred. They live in the spaces between sand and silt grains, and are able to withstand very low levels of oxygen. They lack a pseudocoel, a circulatory system, and an anus, with poorly developed parenchyma. The pharynx is armed with a pair of lateral jaws, used to scrape fungi and bacteria off the surrounding substratum. They can glide, swim in loops and spirals, and bend their heads from side to side. Reproduction is sexual in males, females and hermaphrodites and fertilisation is internal. Pseudocoelomate animals This group consists of nine phyla, and are grouped together as the original blastocoel of the embryo existing as a space (body cavity) between the digestive tract and body wall. Because this cavity lacks the peritoneal cavity in true coelomates it is termed a pseudocoel. This is their main importance biologically, as a pseudocoel allows greater freedom of movement, and space for the differentiation of organ systems. Circulation systems are present, as is a complete mouth to anus digestive system. Ecdysozoan Phyla Phylum Nematoda -- Roundworms So far, 12,000 species have been described, but it is thought there may be up to 500,000 worldwide. Habitats range from marine water, freshwater, and soil, from the tropics to polar, and mountain regions. A peculiarity of the group is that during growth, their cells get larger, but the total number is constant. This is called eutely. Nematodes are also parasites on virtually every plant and animal species. Effects of nematode infestation on crops, domestic animals and humans make this phylum one of the most important of all parasitic animal groups. Free-living nematodes feed on bacteria, yeast, fungal hyphae, faecal material, and plant juices. Predatory species consume rotifers, small annelids and other nematodes. In turn, the group provides food for mites, insect larvae, and certain species of nematode catching fungi. Reproduction is usually sexual. Males are usually smaller than females, and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults. Parasitic forms often have quite complicated life cycles, moving between several different hosts or locations in the host\'s body. Infection occurs variously, by eating uncooked meat with larvae in it, by entrance into unprotected cuts or directly through the skin, by transfer via blood-sucking insects, and so forth. Nematodes commonly parasitic on humans include whipworms, hookworms, pinworms, ascarids, and filarids, e.g. elephantiatis caused by Wuchereria bancrofti. Trichinella spiralis, occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. Trichinosis initially involves the intestines. Within 1 2 days of contagion, (by eating poorly-cooked infected game or pork meat) manifestations such as nausea, heartburn, dyspepsia and diarrhoea occur. Later on, as the worms encyst in different parts of the human body, other symptoms such as headache, fever, chills, cough, eye swelling, joint pain and muscle pain and itching occur. The most dangerous case is worms entering the central nervous system, producing serious neurological deficits (such as ataxia or respiratory paralysis), even death. Haemonchus contortus is one of the most abundant infectious agents in sheep around the world. In contrast, nematodes, which parasitize insects, are considered beneficial. Depending on the species, a nematode may be beneficial or detrimental to a gardener\'s cause. From a horticultural perspective, there are two categories of nematode: predatory ones, which will kill garden pests like cutworms, and pest nematodes, like the root-knot nematode, which attack garden plants. Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control. Phylum Nematomorpha Known as 'horsehair worms', about 250 species have been described. Worldwide in distribution, they are free-living as adults, and parasitic in arthropods as juveniles. Digestive systems are vestigial, and they lack circulatory, respiratory and excretory systems. Phylum Kinorhyncha Small (less than 1mm long) and able to utilise marine habitats from pole to pole and up to 600m depth. Most live in mud flats, but may live in algae holdfasts and sponges. They are divided into 13 segments, and have a retractile head with a small retractile proboscis. They also have as a complete digestive tract and sensory eyespots. Phylum Loricifera Not described until 1983, they are tiny (0.25mm) with a protective external case (lorica). They live in interstitial species between sand grains. Phylum Priapulida Consisting of 18 species and found in colder waters of both hemispheres, ranging from intertidal flats to a depth of several thousand meters. They vary in length from 12cm to 30cm long, and are found in substratum with the mouth uppermost. They usually have small caudal appendages, a chitinous cuticle, and separate sexes. Lophotrochozoan Phyla Phylum Rotifera These animals have a characteristic ciliated 'crown' or corona. They range from 40um to 3mm in length. They are often brightly coloured and may have bizarre shapes correlated to the habitat niche. Floating forms are globular/saclike (planktonic forms). Creepers and swimmers are elongated and worm-like, and sessile forms are usually vase like. There are around 1800 species, found throughout the world, most are freshwater living and some are marine or terrestrial, other live on other animals (epizoic) or are parasitic. One interesting class (Bdelloidea) has no known male form. All individuals are female and reproduce by parthenogenesis. Phylum Acanthocephala Commonly known as 'spiny headed worms', which derive their name from their distinctive feature of a cylindrical, invaginable proboscis, bearing rows of recurving spines, by which they attach themselves to their host. All of the group are endoparasites, living in the intestines of vertebrate animals. Their length ranges from 2mm to 1m, with females being larger than males. None of the 500 known species parasitizes humans. Larvae develop in arthropods, crustaceans or insects, depending on the species. Phylum Gastrotricha Small, ventrally flattened animals 65um-500um long, similar to rotifers, but lack a corona and have typically bristly bodies. The 400 species are found in fresh and marine water, living on the substratum, or animal/plant surfaces. Locomotion is by ventral cilia. Phylum Entoprocta These are small phylum of 150 tiny sessile animals, superficially resembling hydroid cnidarians, but are distinguished by having inward rolling ciliated tentacles. All of them are marine sessile stalked forms, and some of them are colonial. Phylum Mollusca There are some 112,000 species within this phylum. The scientific study of molluscs is called malacology. Molluscs range from minute snails and clams to larger organisms such as squid, cuttlefish and octopus, which are among the most neurologically advanced invertebrates. The vast majority of molluscs live in marine environments, and are found intertidally, in the shallow subtidal and on the continental shelf, although some species do live in the abyssal depths of the oceans around hot vents. Not all molluscs are marine creatures; the bivalves and the gastropods also contain freshwater species. Only the gastropods have representatives that live on land (land snails and slugs). The mollusca are significant, because gaseous exchange does not occur only through the body surface, as in previously described phyla, but also in specialised respiratory organs, in the form of gills and lungs. Most classes have an open circulatory system, with a pumping heart, vessels and blood sinuses, although in the gastropods, the circulatory system is closed (heart, vessels and capillaries). The efficiency of the respiratory and circulatory systems has led to the development of larger forms. Invertebrates have reached their largest size in the cephalopods. The group has a fleshy mantle that, in most cases, secretes a protective shell, and can be modified to perform a number of additional functions. Additional unique features of this group are the radula (rasping tongue like organ), and a muscular foot for locomotion. The group has advanced sensory systems of touch, smell, taste, equilibrium, and vision, which are very highly developed in some cephalopods, which are considered to be the most intelligent invertebrates. Reduced to its simplest form, the Molluscan body plan may be said to consist of a 'head-foot' portion and a visceral mass. The head-foot part is the more active area, consisting of the feeding, cephalic sensory, and locomotary organs. The visceral mass is the part containing digestive, circulatory, respiratory and reproductive functions, and depends mainly on ciliary tracts for its functioning. Two folds of skin, outgrowths of the dorsal body wall, form a protective 'mantle', which encloses a space (mantle cavity) between the mantle and body wall. The mantle cavity contains gills (ctenidia) or a lung, and secretes the shell in those species, which possess one. Greater emphasis on either the head or foot portion, determine the vast array of molluscan forms. The shell of a mollusc, when present, is secreted by and lined with the mantle. Consisting of three layers, the periostracum is the outer horny layer and helps protect the lower layers. The middle layer (peimatic layer) is composed of calcium carbonate, laid down in a protein matrix. The inner nacreous layer is secreted continuously by the mantle and grows in depth throughout the animal's life. It forms the 'mother of pearl' material and actual pearls that are valued for jewellery. Most molluscs are dioecious, although some are hermaphrodite. The free-swimming larvae that emerge from eggs are termed trochophores, which are the ancestral form of annelids. Direct metamorphosis of a trochophore into a juvenile, is regarded as ancestral for molluscs. A higher characteristic is the intervention of another free-swimming larval stage, a veliger (as in many gastropods and bivalves). In many molluscs, the trochophore stage occurs with the egg, with the release of a veliger. Cephalopods and some other forms have no free-swimming larvae, and juveniles hatch directly from eggs. There are ten classes of molluscs, eight are still living, and the others are known only from fossils. These classes make up the 250,000 and more species of mollusc: Class Caudofoveata - (deep-sea, worm-like creatures, 70 known species). Now generally recognised as a subclass of Aplacophora. Burrowing lifestyle, no shells, radula present, thought to be close to ancestral molluscs. Class Solenogastres - (deep-sea, worm like creatures, 250 species). No radula or gills. Live free on the substratum, no burrowing. Class Polyplacophora - (chitons, 600 species, rocky marine shorelines). They are flattened dorsoventrally, and have a concave dorsal surface that has 8 articulating plates. The head and sensory organs are reduced, but these are photosensitive organs, which pierce the plates. They prefer rocky surfaces in intertidal areas. The radula is reinforced with the iron-containing mineral, magnetite, which it uses to scrape algae from rock. Class Monoplacophora - (deep-sea limpet-like creatures, 11 living species). Until 1952 this group was thought extinct, until an example was dredged up off the coast of Costa Rica. They have a low rounded shell and a creeping foot, superficially resembling limpets, but have some serially repeated organs like some Chitons; they have five pairs of gills, six pairs of nephridia and one or two pairs of gonads. Class Bivalvia - (clams, oysters, scallops, mussels, 8000 species). They range in size from 1mm to 1m and are 225kg in weight (giant pacific clam, Tidacna). Most are sedentary filter feeders, depending on cilliary currents, produced by cilia on gills, to bring in food particles. Unlike gastropods they have no head, no radula and very little cephalisation. Bivalves are laterally compressed. The two shells are held together by a hinge ligament, which allows the shell to gape ventrally. The umbo is the oldest part of the shell, as growth occurs in concentric lines around it. Bivalve move by extending the foot between the valves, blood is pumped into the foot, causing it to swell, and acts like an anchor in the substratum. Then, longitudinal muscle contracts and shortens the foot, pulling the animal forward. Some species are able to swim by clapping their shells together, to create a sort of 'jet propulsion'. Sense organs are poorly developed. They include tactile cells, and although the scallop has a row of small blue eyes along the mantle edge, each eye has a cornea, lens and retina. Sexes are usually separate; gametes are discharged into the suprabranchial chamber to be carried out with the excurrent flow. In most bivalves, fertilisation is external, although internal in freshwater clams. Class Scaphopoda - (tusk shells, 350 species, all marine). These are bottom dwelling organisms, and have a tubular shell open at both ends. The foot protrudes through the larger end of the shell and is used to burrow into mud or sand, leaving the smaller end exposed to the water. Food is caught on cilia of the foot, or on ciliated tentacles extending from the head. Sexes are separate and development is through a larval trochophore. Class Gastropoda - (sea snails with shells, such as abalone, limpets, conch, and marine snails without a shell or with a reduced shell, such as nudibranchs, sea hares, sea angel, sea butterfly, sea lemon etc; land snails and slugs, freshwater snails, total estimated at 40,000 - 150,000 species). This is by far the largest and most diverse group. Shells (where present) are in one piece (univalve) and are usually coiled around a central axis. Gastropods range from microscopic forms to Pleuroploca gigantea, which has a shell 60cm across. Fossil examples have been found that are over 2m long. Many gastropods have an operculum, a horny plate that covers the shell aperture when the body is withdrawn into the shell. Before torsion, the embryos gill/anus is posterior, and is growth. After torsion, the anus opens disadvantage of this system is waste products could occur. Of all the Mollusca, only Gastropods undergo torsion. This involves twisting the mantle cavity, which was originally behind the body, and it twists the visceral organs through 90 - 180 degrees. This process occurs at the veliger stage. mouth is anterior, and the brought about by uneven over the head. The that 'fouling' of the gills by However, coiling of the shell has led to the loss of the gill, kidney, and auricle on the side of the main body/shell whorl, leaving a condition of bilateral asymmetry. This arrangement avoids fouling, as water is brought into one side of the mantle cavity and out on the other, carrying with it the waste from the anus (and nephridopore) lying on that side.