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.

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

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.

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.

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).

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).

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.

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).

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).

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).

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).

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).

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).

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.