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Wednesday,May 06, 2009

AFRMS37th Common

Join Date: Mar 2006

Posts: 1,514Thanks: 1,053Thanked 1,670 Times in 871 Posts

Phylum Arthropoda

Phylum Arthropoda

Arthropods are multicellular, triploblastic, bilaterally symmetrical, metamerically segmented,schizocoelous, protostomous invertebrate metazoans.

Cephalization of some anterior segments of the arthropods from the head is present.

Externally the body is covered with a thick tough, non living, chitinous and protective cuticle, formingthe exoskeleton. Exoskeleton is non-living and cannot grow. Appendages are segmental, paired,lateral and jointed and variously modified asjaws, gills and legs etc.Arthropods are triploblastic animals have true coelom.

Arthropods possess separate striated muscles. More than one pair of jaw is present.Circulatory system is open, capillaries are absent and arteries open into irregular spaces calledsinuses. Malpigian tubules or coelomoducts are the excretory organs that excrete ammonia, urates,amines or guanine. Nervous system is of anneidian type. Compound eye with mosaic vision is welldeveloped. Cilia and flagella are entirely absent.

Sexes are usually separate (dioecious), but a few are hermaphrodite. Sexual dimorphism is usuallyevident. Parthenogenesis is common in some groups. Gonads and their ducts usually paired.
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#52

Fertilization is internal and development include complete, incomplete or no metamorphosis. Parentalcare often well marked.

these are two links of PPt presentation about Arthropods

Click here

and

check this

regards

The Following 2 Users Say Thank You to AFRMS For This Useful Post:

Shali (Thursday, October 08, 2009)

Wednesday, May 06, 2009

AFRMS37th Common

Join Date: Mar 2006Posts: 1,514Thanks: 1,053Thanked 1,670 Times in 871 Posts

Arthropoda

Phylum Arthropoda

The following is the link to detailed article on mouth parts of insects on Wikipedia

check this link

The link mentioned below is a comprehensive presentation on mouthparts of insects with fully labelleddiagrams.

click here

regards
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#53


dr.atifrana (Wednesday, May 06, 2009), Shali (Thursday, October 08, 2009)

Wednesday, May 06, 2009

AFRMS37th Common


metamorphosis

Phylum ArthropodaInsect Metamorphosis

Introduction

Metamorphosis refers to a major change of form or structure during development.

Insect Metamorphosis

One of the most dramatic forms of metamorphosis is the change from the immature insect into theadult form. Most of the major insect orders have a typical life cycle which consists of an egg, whichhatches into a larva which feeds, moults and grows larger, pupates, then emerges as an adult insectthat looks very different from the larva. These insects are often called 'Holometabolous', meaningthey undergo a complete (Holo = total) change (metabolous = metamorphosis or change). Thosewhich have immature stages similar in shape to the adult minus the wings are called'Hemimetabolous', meaning they undergo partial or incomplete (Hemi = part) change.

Holometabolous (complete metamophosis)

Typical holometabolous insect groups are the Coleoptera (Beetles), Lepidoptera (moths, butterfliesand skippers), Hymenoptera (sawflies, wasps, ants and bees) and Diptera (flies). All these groupshave a life cycle where the egg hatches into a larva (eg a caterpillar, grub, maggot) which goesthrough an inactive, pupa stage (eg wrapped up like a cocoon) before emerging as an adult (eg abutterfly, beetle, wasp).

Hemimetabolous (incomplete metamorphosis)

Typical hemimetabolous insects are the Hemiptera (Scales, Aphids, Whitefly, Cicadas, Leafhoppersand True Bugs), Orthoptera (Grasshoppers and Crickets), Mantodea (Praying Mantids), Blattodea(Cockroaches), Dermaptera (Earwigs) and Odonata (Dragonflies and Damselflies). These groups gothrough gradual changes as they turn into adults. Immature forms of these insects are called nymphsand these gradually increase in size and change form. As the insect grows, it sheds its skin (calledmoulting). After each moult, the nymph looks a bit different or a bit bigger. After a final moult, thefull adult form emerges.

A successful strategy

Metamorphosis is one of the key elements that explains why insects are so successful. Many insectshave immature stages with completely different habitats from the adults. This means that insects canoften exploit valuable food resources while still being able to disperse into new habitats as wingedadults. The potential for adaptation and evolution is greatly enhanced by metamorphosis.

Growth and maturity
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There is an important feature to note regarding metamorphosis. Insects are not able to mate andreproduce until they undergo their final moult or emerge from a pupa as a winged adult. Wings do notappear until the final moult (the one exception to this is the Ephemeroptera, or Mayflies). When yousee an insect with wings, it is fully grown. This means that small flies do not become larger flies, theyare as big as they will get.

Caterpillars, Grubs and Maggots - Holometabolous Larvae

Holometabolous larvae are larvae that pupate before emerging as adult insects, and include many ofthe most familiar insects.Holometabolous larva in general are little more than tubular, efficient eating machines. They do nothave to lay eggs, or find a mate. Apart from eating, they are mainly concerned with avoiding beingeaten themselves. This means that they may have good camouflage, or hide in shelters or holes, orthey may taste dreadful to any prospective predators.The major insect orders have larvae with different common names. For instance, moths, butterfliesand skippers have larvae which are usually called caterpillars. Fly larvae are nearly always calledmaggots. Beetle larvae are often referred to as grubs.

Caterpillars

Moth, butterfly and skipper (Lepidoptera) caterpillars have pairs of prolegs on their abdomen inaddition to the three pairs of jointed walking legs on the thorax. Prolegs differ from the usual insectlegs in that they are not jointed. Each proleg has a set of tiny hooks, which are arranged in rings orseries around the tip of the proleg. These are called crochets, and only occur in the insect orderLepidoptera. Although there are some caterpillar-like larvae from other insect orders, such as sawflylarvae (Order Hymenoptera, Suborder Symphyta) and leaf beetle larvae (Order Coleoptera, FamilyChrysomelidae), they can be distinguished from lepidopteran larvae by the absence of prolegs withcrochets. Lepidopteran larvae have chewing mouthparts, and the majority of species are adapted toeating plant material

Maggots

Fly larvae (Diptera) lack any segmented legs on the thorax, and are often highly specialised for living

in wet environments. Very few are adapted to dry conditions. Quite a few species are internalparasites of other animals, where legs would be of no use. Unlike the larvae of Lepidoptera there isno one character that can be used to separate fly maggots from other large orders such as theHymenoptera (Wasps, Bees, Ants, and Sawflies), as the immature stages of many species in theseorders also lack segmented legs. Fly maggots live on a huge range of foods - from human fleshthrough to kelp on the seashore

Grubs

Beetle larvae (Coleoptera) are highly diverse in their shapes. The majority live in concealed habitats,such as underground, or inside trees. There are many aquatic species, and a few which resemblecaterpillars and feed openly on leaves. Many retain segmented legs, although weevil grubs nearlyalways lack legs. Most legless beetle grubs have robust chewing mouthparts and can be distinguished

from fly maggots, which often have modified mouth 'hooks'.The larvae of sawflies, wasps, bees and ants (Hymenoptera) are diverse in form. Many sawfly larvaeare similar to lepidopteran caterpillars, and feed externally on plant material. The socialHymenoptera, which includes some wasps, some bees, and all ants have larvae with very fewexternal features, as they do not have to forage for food. In these species food is brought to them bythe adult nest mates. The parasitic Hymenoptera are similar in that they spend their larval periodinside hosts or well-stocked nests. They do not need camouflage or legs in these habitats

Nymphs, hoppers and mudeyes - Hemimetabolous insects

Hemimetabolous insects do not have a pupal stage. The general appearance of the immature stages issomewhat similar to that of adults, although there may be some dramatic differences in lifestyle. Only


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#54

adult insects are able to reproduce, and only adult insects have functional wings (in those species thathave wings).The immature stages of these insects are generally called nymphs rather than larvae. Some havecommon names such as 'hoppers' (immature grasshoppers, Order Orthoptera), 'crawlers' (immaturescale insects, Order Hemiptera) and 'mudeyes' (immature dragonflies, Order Odonata).Examples of hemimetabolous insects include cockroaches (Order Blattodea), crickets andgrasshoppers (Order Orthoptera), stick insects (Order Phasmatodea), praying mantids (OrderMantodea), termites (Order Isoptera), dragonflies and damselflies (Order Odonata), earwigs (OrderDermaptera), sucking bugs (Order Hemiptera), wood and book lice (Order Psocoptera), and parasiticlice (Order Phithaptera).

The feeding habits of hemimetabolous insects commonly mirror that of the adults, but often with asignificant twist. Dragonfly nymphs are aquatic predators, but the adults are active flying insects,which hunt other flying insects. Stick insect nymphs can resemble ants, while later stage nymphsblend with the food plants. All stages of stick insects feed on plant material. The final moult betweenmature nymph and adult is usually accompanied by changes in colour, and in shape of the body, butthere is never the dramatic difference between larvae and adult as observed in holometabolousinsects.

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Thursday, May 07, 2009

AFRMS37th Common


Insect

Phylum Arthropoda

The importance of insects

Insects represent animals commonly found in different types of the environment. They adapted toextremely harsh living conditions by developing modified, and often quite complicated mouth-parts.This helps them use all kinds of available food. It is therefore not surprising, that these widespread

and numerous animals significantly affect the environment in which they live. Insects are also ofgreat importance for the economy. Some of them are our allies whereas others are grimly foughtenemies.One of the most important roles insects play in the natural word is the pollination of flower plants .Over millions of years, the evolution of flower plants and the related insects proceeded in parallel. Asa result, various tools for collecting and transporting pollen have been developed, such as ventralbrushes, pollen-baskets on legs or tufts of hair on other parts of the body. Some species, for instance,have unusually long tongues which help them reach the bottom of elongated flower tubes in search ofnectar. Some insects pollinate flowers blooming in the daytime while others prefer flowers that openat twilight. The most important pollinators of flower plants are hymenopterans, especially wild bees,as well as lepidopterans, dipterans and coleopterans.Numerous insect species compete for food with man, causing considerable damage to crops orconsuming wild plants which are also utilized by people. The chrysomelid beetles (Chrysomelidae)
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#55

feed on green plant tissue. This leads to a significant decrease in the size of yield where the beetlesoccur in large numbers.The most famous representative of this family is the Colorado beetle(Leptinotarsa decemlineata). Other species commonly found in the Park include the red poplar leaf-beetle (Chrysomela populi), which feeds on poplars, willows and aspen, as well as one of the spottedleaf beetles, Chrysomela viginitipunctata.Insects which exercise the greatest influence on tree stands are the bark beetles (Ipidae). Theseminiature beetles can cause the withering of large forest areas already weakened by air pollution orsevere weather conditions such as long-lasting drought. Bark beetles live mainly in wood and underthe bark, the traces of boring of which are characteristic of individual species. Most changes in thespruce stands of the Wigry National Park have for years been caused by the eight-dentated bark

beetle (Ips typographus).Tree stands are destroyed also by some Hymenoptera species. The mass occurrence of phytophagushymenopterans (Symphyta) such as the pine web-spinning sawfly (Acantholyda posticalis) or the pinesawfly (Diprion pini), can severely damage the assimilation organs in trees (caterpillars feed onneedles) and significantly decrease their immunity to possible attacks of other insects (e.g. one ofbuprestid beetles, Phaenops cyanes) or fungi, presenting as they do a considerable danger to entiretree stands.A small group of insects feeds on the blood of warm-blooded animals. This way of acquiring food ischaracteristic of certain dipterans such as keds (Melophagus), fowl flies (Ornithomyia), horse flies(Tabanus) and deer flies (Chrysopus), as well as various bugs (e.g. bed bug). Blood-sucking dipteransmay transmit many diseases. Horse flies and deer flies may carry rabbit fever and plague germs bysucking the blood of farm animals.In spite of their arduousness as well as negative impact on the environment of man, insects play an

important role in the natural world. Many species of predatory and parasitic insects significantlyreduce the number of organisms which are harmful to the human economy . These insects regulateand maintain the biocoenotic balance and ensure in this way a proper functioning of the naturalenvironment. This insect group includes, among others, all carabid species (Carabus). The larvae ofmost ladybird species (Coccinellidae) (e.g. two-spot lady-bird) play a similar role in nature bydevouring enormous amounts of aphids, scale insects and other tiny insects. Some species of the rovebeetles family (Staphylinidae) penetrate the corridors of bark beetles in search of their larvae. Asimilar behaviour is typical of some representatives of the chequered beetle family (Cleridae),including the ant beetle (Thanasismus formicarius) inhabiting the park.Another group of insects which plays a crucial role in different types of forest environment is ants(Formicidae). Large mound ants belonging to the Formica genus act as "orderlies" by regulating thenumber of other insects. In the case of the mass appearance of Lepidoptera or Diptera caterpillarsfeeding on plants, ants switch to these species thereby significantly reducing their number. By buildingtheir nests, ants improve the quality of the soil. Numerous chambers and corridors in the underground

part of the nest have a beneficial impact on the air and water regime in the soil.Insects actively accelerate the circulation of the organic matter in the environment. The larvae of

many Diptera species (e.g. bluebottle flies Caliphora and flesh-flies Sarcophaga) feed on dead plantsand animals as well as on animal dung. This significant contribution leads to a faster decomposition.Carrion is a source of nourishment for numerous beetle species (e.g. burying beetles (Necrophorus)and carrion beetles Silpha). Because eggs are deposited in the carrion, the larvae feed on the animalremains. The dor beetles (Geotrupes) remove immense quantities of dung from the environment .They build deep burrows ending with chambers under an accumulation of dung where females deposittheir eggs. The beetles then fill the chambers with lumps of dung providing food reserves for thedeveloping larvae

The Following User Says Thank You to AFRMS For This Useful Post:Shali (Thursday, October 08, 2009)

Thursday, May 07, 2009

AFRMS37th Common


crustaceans
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#56

Phylum Arthropoda

General features Importance to humans

The crustaceans of most obvious importance to humans are the larger species, chiefly decapods.Fisheries in many parts of the world capture shrimps, prawns, spiny lobsters, and the king crab(Paralithodes) of the northern Pacific and its southern counterpart, the centolla, found off the coast ofChile. Many species of true crabssuch as the blue crab, Dungeness crab, and the stone crab, all inNorth America, and the edible crab of Europeare valuable sources of food. The most highly prizeddecapod is probably the true lobster (Homarus species), although overfishing since the early 20th

century has greatly diminished the catches of both the North American and the European species.Freshwater crustaceans include crayfish and some river prawns and river crabs. Many species haveonly local market value. It is probable that no crustaceans are poisonous unless they have beenfeeding on the leaves or fruits of poisonous plants.

Another crustacean, the large acorn shell (Balanus psittacus), a barnacle (order Cirripedia) measuringup to 27 centimetres (11 inches) in length, is regarded as a delicacy in South America, and a stalkedbarnacle (Mitella pollicipes) is eaten in parts of France and Spain. In Japan, barnacles are allowed tosettle and grow on bamboo stakes, later to be scraped off and crushed for use as fertilizer.Copepods and krill are important components of most marine food webs. Planktonic (i.e., drifting)copepods, such as Calanus, and members of the order Euphausiacea (euphausiids), or krill, may bepresent in such great numbers that they discolour large areas of the open sea, thus indicating tofishermen where shoals of herring and mackerel are likely to be found.

The water flea (Daphnia magna) and the brine shrimp (Artemia salina) are used as fish food inaquariums and fish ponds, and the larvae of the latter are widely used as food for the larvae of largercrustaceans reared in captivity. Ostracods, of which numerous fossil and subfossil species are known,are important to geologists and oil prospectors.Much damage may be done to rice paddies by burrowing crabs of various species and by the mud-eating, shrimplike Thalassina of Malaya. By undermining paddy embankments, they allow water todrain away, thus exposing the roots of the plants to the sun if near the coast, salt water may thus beallowed to seep into the paddies. Tadpole shrimps (Triops) are often numerous in rice fields, wherethey stir up the fine silt in search of food, killing many of the plants. Land crabs and crayfish maydamage tomato and cotton crops.

Reference:Britannica


dr.atifrana (Friday, May 08, 2009), Shali (Thursday, October 08, 2009)

Friday, May 08, 2009

AFRMS37th Common


Respiration in insects

Respiration in Insects

All insects are aerobic organisms -- they must obtain oxygen (O2) from their environment in order tosurvive. They use the same metabolic reactions as other animals (glycolysis, Kreb's cycle, and theelectron transport system) to convert nutrients (e.g. sugars) into the chemical bond energy of ATP.During the final step of this process, oxygen atoms react with hydrogen ions to produce water,releasing energy that is captured in a phosphate bond of ATP.

The respiratory system is responsible for delivering sufficient oxygen to all cells of the body and forremoving carbon dioxide (CO2) that is produced as a waste product of cellular respiration. Therespiratory system of insects (and many other arthropods) is separate from the circulatory system. It
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is a complex network of tubes (called a tracheal system) that delivers oxygen-containing air toevery cell of the body.

Air enters the insect's body through valve-like openings in the exoskeleton. These openings (calledspiracles) are located laterally along the thorax and abdomen of most insects -- usually one pair ofspiracles per body segment. Air flow is regulated by small muscles that operate one or two flap-likevalves within each spiracle -- contracting to close the spiracle, or relaxing to open it.

After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusingthroughout a complex, branching network of tracheal tubes that subdivides into smaller and smallerdiameters and reaches every part of the body. At the end of each tracheal branch, a special cell (the

tracheole) provides a thin, moist interface for the exchange of gasses between atmospheric air and aliving cell. Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffusesinto the cytoplasm of an adjacent cell. At the same time, carbon dioxide, produced as a waste productof cellular respiration, diffuses out of the cell and, eventually, out of the body through the trachealsystem.

Each tracheal tube develops as an invagination of the ectoderm during embryonic development. Toprevent its collapse under pressure, a thin, reinforcing "wire" of cuticle (the taenidia) winds spirallythrough the membranous wall. This design (similar in structure to a heater hose on an automobile oran exhaust duct on a clothes dryer) gives tracheal tubes the ability to flex and stretch withoutdeveloping kinks that might restrict air flow.

The absence of taenidia in certain parts of the tracheal system allows the formation of collapsible air

sacs, balloon-like structures that may store a reserve of air. In dry terrestrial environments, thistemporary air supply allows an insect to conserve water by closing its spiracles during periods of highevaporative stress. Aquatic insects consume the stored air while under water or use it to regulatebuoyancy. During a molt, air sacs fill and enlarge as the insect breaks free of the old exoskeleton andexpands a new one. Between molts, the air sacs provide room for new growth -- shrinking in volumeas they are compressed by expansion of internal organs.

Small insects rely almost exclusively on passive diffusion and physical activity for the movement ofgasses within the tracheal system. However, larger insects may require active ventilation of thetracheal system (especially when active or under heat stress). They accomplish this by opening somespiracles and closing others while using abdominal muscles to alternately expand and contract bodyvolume. Although these pulsating movements flush air from one end of the body to the other throughthe longitudinal tracheal trunks, diffusion is still important for distributing oxygen to individual cellsthrough the network of smaller tracheal tubes. In fact, the rate of gas diffusion is regarded as one of

the main limiting factors (along with weight of the exoskeleton) that prevents real insects fromgrowing as large as the ones we see in horror movies!
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dr.atifrana (Friday, May 08, 2009), Shali (Thursday, October 08, 2009)
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#57Friday, May 08, 2009

AFRMS37th Common


Arthropoda appendages

Arthropoda appendages

Arthropods are unusual among invertebrates they lack locomotory cilia, even as larvae. The problemthat a rigid external covering imposes on movement has been solved by having the exoskeleton dividedinto plates over the body and through a series of cylinders around the appendages. At the junction, or

joints, between the plates and cylinders the exoskeleton is thin and flexible because it lacks theexocuticle and because it is folded. The folds provide additional surface area as the joints are bent. Thearthropods exoskeleton is therefore somewhat analogous to the armour encasing a medieval knight.Most arthropods move by means of their segmental appendages, and the exoskeleton and the muscles,which attach to the inside of the skeleton, act together as a lever system, as is also true invertebrates. The external skeleton of arthropods is a highly efficient system for small animals. Theexoskeleton provides a large surface area for the attachment of muscles and, in addition to functioningin support and movement, also provides protection from the external environment. The cylindrical

design resists bending, and only a relatively small amount of skeletal material need be invested inthickness to prevent buckling. The external skeleton imposes limits on the maximum size of anarthropod, especially in those that live on land. The largest arthropods live in the sea, where they gainconsiderable support from the buoyance of seawater. On land, an excessive amount of skeleton wouldbe required to support a large bulk and, in addition, the new soft skeleton might collapse following amolt.Appendages of arthropods have been adapted for all types of locomotionwalking, pushing, running,swimming, and burrowing. In most arthropods the legs move alternately on the two sides of the bodyi.e., when one leg is in a power stroke, its mate on the opposite side of the body is in the recoverystroke (the same is true of mammals when walking). The legs in front or back are a little ahead orbehind in the movement sequence. Because of the lateral position of the legs, the body of an arthropodtends to hang between them. Leg interference and trunk wobble tend to be problems in an animal witha long trunk and many legs, such as a millipede or centipede. Most arthropods have evolved more

compact bodies and a smaller number of legs. The number of pairs of legs used in walking is not morethan seven (crustacean pill bugs), four or five (shrimps and crabs), four (arachnids), and three(insects). This reduces the problem of mechanical interference. When a ghost crab, for example, isrunning rapidly across a beach or dune, only the second, third, and fourth pairs of the five pairs of legs(counting the claws) are employed. Leg interference is further reduced in most arthropods by varyinglimb length and placement. For example, in Scutigera, the centipede commonly seen in houses, thelegs increase in length from front to back and thus pass over or under one another in stepping. Thetendency for the trunk to wobble has been reduced in some centipedes by having overlapping dorsalplates and in millipedes by having pairs of segments fused to form double segments. Many arthropodsare capable of walking on vertical surfaces. Some simply grip minute surface irregularities with theclaws at the end of the legs. Others, such as certain spiders and flies, have an array of specializedgripping hairs at the ends of the legs.Insect wings are not segmental appendages as are the legs. The paired wings arise as lateral folds of

the integument, one pair above each of the last two pairs of legs. Each wing thus consists of an upperand lower sheet of exoskeleton closely applied to each other. The two skeletal sheets are separated atvarious places, forming tubular supporting veins. Unlike the wings of an airplane, the wings of insectsare flat plates, and lift is obtained by changing the angle at which the front margin of the wing meetsthe oncoming air stream. The evolution of flight is one of several adaptations that have enabled insectsto become the most diverse and populous group of terrestrial animals.A burrowing habit has evolved in some insects, such as mole crickets and ants, but the largestburrowers are crustaceans. Mole crabs and box crabs are rapid burrowers in soft marine sands, andvarious species of mantis shrimps, mud shrimps, and snapping shrimps create elaborate burrows belowthe bottom surface. Crustaceans also include the largest number of arthropod tube dwellers, surpassedonly by certain marine worms (polychaetes). Most of the tube-dwelling crustaceans are amphipods.Their tubes are usually composed of sand or mud particles secreted together and attached to bottomobjects there are, however, some amphipods that carry their tubes with them like a portable house.
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Reference:Britannica.


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#58

Last edited by AFRMS Friday, May 08, 2009 at 06:15 PM. Reason: formatting



Saturday, May 09, 2009

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Phylum Echinodermata


Introduction

Echinoderms form a well-defined and highly-derived clade of metazoans. They have attracted muchattention due to their extensive fossil record, ecological importance in the marine realm, intriguingadult morphology, unusual biomechanical properties, and experimentally manipulable embryos. Theapproximately 7,000 species of extant echinoderms fall into five well-defined clades: Crinoidea (sealilies and feather stars), Ophiuroidea (basket stars and brittle stars), Asteroidea (starfishes),Echinoidea (sea urchins, sand dollars, and sea biscuits), and Holothuroidea (sea cucumbers). Thephylogenetic position of the Concentricycloidea (sea daisies 2 species), remains controversial (Bakeret al. 1986 Smith 1988b Pearse and Pearse 1994 Mooi et al. 1997).Approximately 13,000 echinoderm species are known from the fossil record. All Mesozoic andCenozoic forms clearly fall into the five extant clades, but the Paleozoic record contains numerousdistinct and often bizarre forms that have been placed into approximately 15 additional classes.Phylogenetic relationships, and in some cases status as monophyletic groups, remains unclear for theextinct classes. Unquestionable echinoderms first appear in the fossil record during the mid-Cambrian.


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Arkarua, a possible echinoderm, has been described from the Vendian (latest Proterozoic) (Gehling1987).

Characteristics

Synapomorphies of the Echinodermata

Echinoderms are among the most distinctive of all animal phyla. Inclusion in the phylum is readilydiagnosable on basis of the four synapomorphies below. Most of these features are present, or can beinferred, even in the earliest fossils. Together, these synapomorphies define much of what makes thefunctional biology of echinoderms distinctive from that of other metazoans.Calcitic skeleton composed of many ossicles.

The biomineral matrix of echinoderm skeletons is composed of calcium carbonate and severalproteins. The calcite is deposited as numerous tiny crystals, but all of them lie on the same crystalaxis within an ossicle. For this reason, ossicles are birefringent under polarizing light. Ossicles are notsolid, but have a sponge-like microstructure called stereom that is unique to the phylum.Embryologically, echinoderm ossicles are a true endoskeleton, since they are produced bymesenchymal cells and are usually covered by epidermis. Functionally, however, the majority of

ossicles act more like an exoskeleton, lying just under the epidermis and enclosing most other tissuesin a flexible but tough covering.

Water vascular system.

The water vascular system performs many important functions in echinoderms, including locomotion,respiration, and feeding in addition, most sensory neurons are located at the termini of podia(tubefeet) which are part of this organ system. The water vascular system may have evolved fromsimple tentacular systems similar to those in other deuterostome phyla, such as the tentacles ofpterobranch hemichordates. However, there are many derived features of the water vascular systemin echinoderms, including: an embryological origin from left mesocoel, podia arranged along branches(ambulacra), and a central circumesophageal ring.

Mutable collagenous tissue.

The ossicles of echinoderms are connected by ligaments composed predominantly of collagen. Thematerial properties of this connective tissue are mutable on short timescales, under neuronal control.Ligaments are normally "locked" (rigid), but can be temporarily "unlocked" (loosened). This providessome interesting mechanical advantages, including the ability to maintain a variety of postures withno muscular effort. In holothuroids, which contain only microscopic ossicles, the entire body wallcontains mutable collagenous tissue.

Pentaradial body organization in adults.

The adults of all extant echinoderms are radially symmetrical. A superficial bilateral organization hasevolved twice, in irregular echinoids and holothuroids, but is based on an underlying five-foldorganization of skeleton and most organ systems, and is clearly secondary. Higher order radial

symmetry (e.g., seven-fold or nine-fold) has evolved on several occasions, and is also clearly asecondary modification. The evolutionary origins of five-fold symmetry remain obscure. Some earlyPaleozoic echinoderms are not radially symmetrical (e.g., carpoids and helicoplacoids), while apossible echinoderm from the Vendian (Arkarua) has five-fold radial body organization.

Plesiomorphies and other featuresMarine habit.

All extant echinoderms live in the ocean, and there is no fossil evidence of any exception to this.Within the marine realm, echinoderms occupy nearly all habitats, where they often constitute a majorproportion of the biomass.


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Pelago-benthic life cycle.With rare exception, echinoderms are gonochoric (separate sexes) with no overt sexual dimorphism.Fertilization is almost always external. Ancestrally (and still, typically), the life cycle is complex, witha free-living larva that is planktotrophic (grazes on unicellular algae). Larvae are plesiomorphicallybilaterally symmetrical, have a recurved gut and transparent ectoderm, and feed by upstream particlecapture using the ciliated band. Metamorphosis is typically radical and occurs during settlement ontothe benthos.

Coelomate.

Echinoderms form their coeloms as outpocketings from the archenteron (embryonic gut), a processcalled enterocoely. In most species, the coeloms are trimerous, and initially bilaterally symmetrical.The fates of the various coelomic compartments vary among echinoderms, but some features seembroadly similar and may reflect a common evolutionary origin deep within the phylum: left mesocoelgives rise to most or all of the water vascular system, and one or both somatocoels form the lining ofthe body cavity.

Deuterostome.

Like some related phyla, the blastopore (site where gastrulation begins) in echinoderm embryosbecomes the larval anus the larval mouth is a secondary opening. In some extant forms, the larvalmouth is preserved as the adult mouth, while in others the entire digestive system is re-plumbedduring metamorphosis and a new mouth and anus form.

Simple hemal/excretory system.The hemal and excretory systems of echinoderms are linked into what Nielsen (1996) calls the "axialcomplex". This organ system shows similarities, and may be homologous, to those of otherdeuterostome phyla. In echinoderms, it is composed of: a thickened vessel (the "heart") lacking anendothelium and surrounded by a pericardium a region where ultrafiltration occurs via podocytes aclosed circulatory system and an opening to the external environment called the madreporite.

Decentralized nervous system.The arrangment of the central nervous sytem of echinoderms is quite different from that in other

deuterostomes. Radial nerves run under each of the ambulacra, and contain the cell bodies of almostall motor neurons and interneurons. A central nerve ring surrounds the gut, and is composed primarilyof fiber tracks connecting the radial nerves. No known echinoderm contains anything that could becalled a brain, although ganglia are present along the radial nerves in some echinoderms. Unlike mostbilaterian phyla, echinoderms lack any trace of cephalization, and have no specialized sense organs.Sensory neurons are located primarily within the ectoderm of podia, and send axons to the radialnerves.


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#59

#60

The Following User Says Thank You to AFRMS For This Useful Post:Shali (Thursday, October 08, 2009)


AFRMS37th Common




The following link is about Echinodermata,characters,classes,water vascular system.

Click Here

Another link to ppt presentation about Echinodermata.

Check this

regards




AFRMS37th Common




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