Animal Kingdom: Non-Chordates

Levels of Organization:

  • Though all members of %%Animalia are multicellular, all of them do not exhibit the same pattern of organization of cells.%%
    • For example, %%in sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit a cellular level of organization.%%
  • Some division of labor (activities) occurs among the cells.
    • %%In coelenterates, the arrangement of cells is more complex.%%
    • Here the cells performing the same function are arranged into tissues, hence is called %%tissue level of organization.%%
  • A still higher level of organization, i.e., organ level is exhibited by members of %%Platyhelminthes and other higher phyla where tissues are grouped together to form organs%%, each specialized for a particular function.
    • In animals like %%Annelids, Arthropods, Molluscs, Echinoderms, and Chordates, organs have associated to form functional systems%%, each system concerned with a specific physiological function.
    • This pattern is called the %%organ system level of organization.%%
    • Organ systems in different groups of animals exhibit various patterns of complexities.
      • For example, %%the digestive system in Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus and is hence called incomplete.%%
    • A complete digestive system has %%two openings, the mouth, and anus.%%
  • Similarly, the circulatory system may be of two types:
    • open type in which the %%blood is pumped out of the heart and the cells and tissues are directly bathed in it%%
    • closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins, and capillaries).

Symmetry:

  • Animals can be categorized on the basis of their symmetry.

    • Sponges are mostly asymmetrical, i.e., any plane that passes through the center does not divide them into equal halves.
    • When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry.
    • Coelenterates, ctenophores, and echinoderms have this kind of body plan.
    • Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.

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Diploblastic and Triploblastic Organisation:

  • Animals in which the cells are arranged in two embryonic layers, external ectoderm, and internal endoderm, are called diploblastic animals.
    • E.g., coelenterates.
  • An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm.
  • Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals

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Coelom:

  • The presence or absence of a cavity between the body wall and the gut wall is very important in classification.
  • The body cavity, which is lined by mesoderm is called coelom. Animals possessing coelom are called coelomates
    • E.g., annelids, mollusks, arthropods, echinoderms, hemichordates, and chordates.
  • In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm.
  • Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates.
    • E.g., aschelminthes.
  • The animals in which the body cavity is absent are called acoelomates.
    • E.g., Platyhelminthes.

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Segmentation:

  • In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs.
    • For example, in earthworms, the body shows this pattern called metameric segmentation, and the phenomenon is known as metamerism.

Notochord:

  • The notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
    • Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates.
    • E.g., porifera to echinoderms.

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Classification of Animals:

Phylum Porifera:

  • Members of this phylum are commonly known as sponges.
    • They are generally marine and mostly asymmetrical animals.
    • These are primitive multicellular animals and have a cellular level of organization.
    • Sponges have a water transport or canal system.
    • Water enters through minute pores (ostia) in the body wall into a central cavity, spongocoel, from where it goes out through the osculum.
      • This pathway of water transport is helpful in food gathering, respiratory exchange, and removal of waste.
    • Choanocytes or collar cells line the spongocoel and the canals.
    • Digestion is intracellular.
    • The body is supported by a skeleton made up of spicules or spongin fibers.
    • Sexes are not separate (hermaphrodite), i.e., eggs and sperm are produced by the same individual.
    • Sponges reproduce asexually by fragmentation and sexually by the formation of gametes.
    • Fertilization is internal and development is indirect having a larval stage that is morphologically distinct from the adult.
  • Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).

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Phylum – Coelenterata (Cnidaria):

  • They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals.
    • The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body.
    • Cnidoblasts are used for anchorage, defense and for the capture of prey.
    • Cnidarians exhibit tissue level of organisation and are diploblastic.
    • They have a central gastro-vascular cavity with a single opening, mouth on hypostome.
    • Digestion is extracellular and intracellular.
    • Some of the cnidarians
    • E.g., corals have a skeleton composed of calcium carbonate.
    • Cnidarians exhibit two basic body forms called polyp and medusa.
    • The former is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.
    • Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
  • Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

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Phylum – Ctenophora:

  • Ctenophores, commonly known as sea walnuts or comb jellies are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organisation.
    • The body bears eight external rows of ciliated comb plates, which help in locomotion.
    • Digestion is both extracellular and intracellular.
    • Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
    • Sexes are not separate.
    • Reproduction takes place only by sexual means.
    • Fertilisation is external with indirect development.
  • Examples: Pleurobrachia and Ctenoplana.

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Phylum – Platyhelminthes:

  • They have dorso-ventrally flattened body, hence are called flatworms.
    • These are mostly endoparasites found in animals including human beings.
    • Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation.
    • Hooks and suckers are present in the parasitic forms.
    • Some of them absorb nutrients from the host directly through their body surface.
    • Specialised cells called flame cells help in osmoregulation and excretion.
    • Sexes are not separate.
    • Fertilisation is internal and development is through many larval stages.
    • Some members like Planaria possess high regeneration capacity.
  • Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

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Phylum – Aschelminthes:

  • The body of the aschelminthes is circular in cross-section, hence, the name roundworms.
    • They may be freeliving, aquatic and terrestrial or parasitic in plants and animals.
    • Roundworms have organ-system level of body organisation.
    • They are bilaterally symmetrical, triploblastic and pseudocoelomate animals.
    • Alimentary canal is complete with a welldeveloped muscular pharynx.
    • An excretory tube removes body wastes from the body cavity through the excretory pore.
    • Sexes are separate (dioecious), i.e., males and females are distinct.
    • Often females are longer than males.
    • Fertilisation is internal and development may be direct (the young ones resemble the adult) or indirect.
  • Examples : Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

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Phylum – Annelida:

  • They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.
    • They exhibit organ-system level of body organisation and bilateral symmetry.
    • They are triploblastic, metamerically segmented and coelomate animals.
    • Their body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida.
    • They possess longitudinal and circular muscles which help in locomotion.
    • Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
    • A closed circulatory system is present.
    • Nephridia help in osmoregulation and excretion.
    • Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
    • Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious.
    • Reproduction is sexual.
  • Examples : Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).

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Phylum – Arthropoda:

  • This is the largest phylum of Animalia which includes insects.
    • Over two-thirds of all named species on earth are arthropods.
    • They have organ-system level of organisation.
    • They are bilaterally symmetrical, triploblastic, segmented and coelomate animals.
    • The body of arthropods is covered by chitinous exoskeleton.
    • The body consists of head, thorax and abdomen.
    • They have jointed appendages (arthros-joint, poda-appendages).
    • Respiratory organs are gills, book gills, book lungs or tracheal system.
    • Circulatory system is of open type.
    • Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
    • Excretion takes place through malpighian tubules.
    • They are mostly dioecious.
    • Fertilisation is usually internal.
    • They are mostly oviparous.
    • Development may be direct or indirect.
  • Examples: Economically important insects – Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect) Vectors – Anopheles, Culex and Aedes (Mosquitoes) Gregarious pest – Locusta (Locust) Living fossil – Limulus (King crab).

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Phylum – Mollusca:

  • This is the second largest animal phylum.
    • Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organisation.
    • They are bilaterally symmetrical, triploblastic and coelomate animals.
    • Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump.
    • A soft and spongy layer of skin forms a mantle over the visceral hump.
    • The space between the hump and the mantle is called the mantle cavity in which feather like gills are present.
    • They have respiratory and excretory functions.
    • The anterior head region has sensory tentacles.
    • The mouth contains a file-like rasping organ for feeding, called radula.
    • They are usually dioecious and oviparous with indirect development.
  • Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Seahare), Dentalium (Tusk shell) and Chaetopleura (Chiton).

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Phylum – Echinodermata:

  • These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied).
    • All are marine with organ-system level of organisation.
    • The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
    • They are triploblastic and coelomate animals.
    • Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side.
    • The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration.
    • An excretory system is absent.
    • Sexes are separate.
    • Reproduction is sexual.
    • Fertilisation is usually external.
    • Development is indirect with free-swimming larva.
  • Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).

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Phylum – Hemichordata:

  • Hemichordata was earlier considered as a sub-phylum under phylum Chordata.
    • But now it is placed as a separate phylum under non-Chordata.
    • Hemichordates have a rudimentary structure in the collar region called stomochord, a structure similar to a notochord.
    • This phylum consists of a small group of worm-like marine animals with the organ-system level of organization.
    • They are bilaterally symmetrical, triploblastic, and coelomate animals.
    • The body is cylindrical and is composed of an anterior proboscis, a collar, and a long trunk.
    • The circulatory system is of open type.
    • Respiration takes place through the gills.
    • Excretory organ is proboscis gland.
    • Sexes are separate.
    • Fertilisation is external.
    • Development is indirect.
  • Examples: Balanoglossus and Saccoglossus.

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