Classification of Animal Kingdom (Animalia)

Basis for Animal Kingdom Classification

Classification of Animal Kingdom is based on various fundamental features like –

1. Levels of Organisation,
2. Symmetry,
3. Diploblastic and Triploblastic Organisation,
4. Coelom development,
5. Segmentation of the body and
6. Presense or absence of Notochord.

• The broad classification of Animalia based on common fundamental features:

Levels of Organisation

• Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells.
• For example, in sponges, the cells are arranged as loose cell aggregates, i.e., they exhibit cellular level of organisation. Some division of labour (activities) occur 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 organisation.
• A still higher level of organisation, i.e., organ level [organ level of organisation] is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised 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 organ system level of organisation.
• Organ systems in different groups of animals exhibit various patterns of complexities.
• For example, the digestive system in Platyhelminthes (incomplete digestive system) 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, 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 and closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).

Symmetry

• Animals can be categorised on the basis of their symmetry.
• Sponges are mostly asymmetrical, i.e., any plane that passes through the centre 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, an external ectoderm and an 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 (platyhelminthes to chordates).

Figure: Showing germinal layers : (a) Diploblastic (b) Triploblastic

Coelom

• 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, Molluscs, Arthropods, Echinoderms, Hemichordates & 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 earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

Notochord

• Notochord is a mesodermally [the middle layer of cells or tissues of an embryo, or the parts derived from this (e.g. cartilage, muscles, and bone)] derived rod-like structure formed on the dorsal side [posterior] 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 Animal Kingdom

Animal Kingdom is classified into:

1. Phylum – Porifera
2. Phylum – Coelenterata (Cnidaria)
3. Phylum – Ctenophora
4. Phylum – Platyhelminthes
5. Phylum – Aschelminthes (Nemotoda) Annelida
6. Phylum – Arthropoda
7. Phylum – Mollusca
8. Phylum – Echinodermata
9. Phylum – Hemichordata
10. Phylum – Chordata

Phylum – Porifera

• Phylum – Porifera includes organisms with holes.
• They are primitive multicellular animals and have cellular level of organisation.
• They are non-motile animals attached to some solid support.
• The body design involves very minimal differentiation and division into tissues.
• They are commonly called sponges.
• They are generally marine and mostly asymmetrical animals.
• 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.
• The body is supported by a skeleton made up of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite), i.e., eggs and sperms are produced by the same individual.
• Sponges reproduce asexually by fragmentation and sexually by formation of gametes.
• Fertilisation is internal and development is indirect having a larval stage which is morphologically distinct from the adult.

Figure: Examples of Porifera : (a) Sycon (b) Euspongia (c) Spongilla

• Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).

Phylum – Coelenterata (Cnidaria)

• The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocytes) present on the tentacles and the body.
• Cnidoblasts are used for anchorage, defense and for the capture of prey.
• Coelenterata (Cnidaria) are aquatic, mostly marine sessile or free-swimming radially symmetrical
• They exhibit tissue level of organization [have more body design differentiation than sponges].
• They have a central gastro-vascular cavity with a single opening.
• They are diploblastic.
• Some of these species live in colonies (corals).
• Some have a solitary [living alone] like–span (hydra).
• 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 (Sea anemone), 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).
• Jellyfish and sea anemones are common examples.
• Digestion is extracellular and intracellular.
• Examples: Aurelia (jelly fish), Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

Phylum – Ctenophora

• Ctenophora are commonly known as sea walnuts or comb jellies.
• They exclusively marine, radially symmetrical, diploblastic
• They exhinit 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 and reproduction takes place only by sexual means.
• Fertilisation is external [fertilization occurs outside the body] with indirect development [zygote → larvae → animal].
• Examples: Pleurobrachia and Ctenoplana.

Phylum – Platyhelminthes

• Platyhelminthes are more complexly designed than the earlier groups.
• They are bilaterally symmetrical.
• They are triploblastic. This allows outside and inside body linings as well as some organs to be made. There is thus some degree of tissue formation [organ level of organisation].
• The body is flattened dorsiventrally, meaning from top to bottom, which is why these animals are called flatworms.
• They may be freeliving or parasitic. Hooks and suckers are present in the parasitic forms.
• Some examples are freeliving animals like planarians, or parasitic animals like
• Parisites are mostly endoparasites found in animals including human beings. Some of them absorb nutrients from the host directly through their body surface.
• Acoelomate: There is no true internal body cavity or coelom, in which well developed organs can be accommodated.
• Specialised cells called flame cells help in osmoregulation and excretion.
• Sexes are not separate.
• Fertilisation is internal and development is indirect.
• Some members like Planaria possess high regeneration capacity.

Phylum – Aschelminthes (Nemotoda)

• Body in aschelminthes (Nemotoda) is cylindrical [bilaterally symmetrical] rather than flattened.
• They exhibit organ-system level of body organization [there are tissues, but no real organs].
• They are triploblastic. A sort of body cavity or a pseudocoelom, is present.
• They are freeliving, aquatic, terrestrial or parasitic in plants and animals.
• These are very familiar as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (roundworm or pinworms).
• The body is circular in cross-section, hence, the name roundworms.
• Alimentary canal is complete.
• 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.

Phylum – Annelida

• Annelida are aquatic [marine and fresh water] or terrestrial; free-living, and sometimes parasitic.
• Their body surface is distinctly marked out into segments or metameres [metamerically segmented] and, hence, the phylum name Annelida (Latin, annulus: little ring).
• They exhibit organ-system level of body organization.
• They are coelomate [true body cavity]. This allows true organs to be packaged in the body structure.
• They are bilateral symmetric and triploblastic.
• 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 (sing. nephridium) 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 [Sexes are separate], but earthworms and leeches are monoecious [having both the male and female reproductive organs in the same individual].
• Reproduction is sexual.

Phylum – Arthropoda

• Insects, arachnids and crustaceans are members of the largest category of creatures on the planet: arthropods.
• Arthropods have hard, external shells called “exoskeletons,” segmented bodies and jointed legs.
• Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions and crabs and some
• They exhibit organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic, segmented and coelomate The coelomic cavity is blood-filled.
• The body of arthropods is covered by chitinous The body consists of head, thorax and abdomen.
• There is an open circulatory system, and so the blood does not flow in well defined blood vessels.
• Respiratory organs are gills, book gills, book lungs or tracheal system.
• Sensory organs like antennae, eyes (compound and simple), statocysts or balance 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.

Arachnids

• Spiders, harvestmen, mites, ticks and other arachnids are members of the class Arachnida.

Crustaceans

• Crustaceans make up a large group of arthropods that includes animals such as crabs, lobsters, crayfish and shrimp. They breathe with gills and have two pairs of antennae.

Insects

• In general, insects have three-part bodies, six jointed legs, compound eyes and two antennae.
• Bees, wasps, beetles, mosquitoes, flies, grasshoppers, ants, butterflies and moths, and dragonflies and damselflies are common types of insects.

Phylum – Mollusca

• Mollusca are the second largest animal phylum. They are terrestrial or aquatic.
• They exhibit organ-system level of organization.
• They are bilaterally symmetrical, triploblastic, coelomate animals. There is little segmentation.
• They have an open circulatory system and kidney-like organs for excretion. 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.
• 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.
• Examples are octopus, snails and mussels.

Phylum – Echinodermata

• These animals have an endoskeleton of calcareous ossicles [calcium carbonate structures] and, hence, the name Echinodermata (spiny skinned organisms).
• They are exclusively free-living marine animals with organ-system level of organisation.
• They are triploblastic with a coelomic cavity [coelomate animals]. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
• Water-driven tube system [water vascular system] are used for locomotion, capture and transport of food and respiration.
• They are triploblastic and coelomate animals.
• Digestive system is complete. An excretory system is absent.
• Sexes are separate. Reproduction is sexual. Fertilisation is usually external.
• Development is indirect with free-swimming larva.
• Examples: Star fish, Sea urchin, Sea lily, Sea cucumber, 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.
• This phylum consists of a small group of worm-like marine animals with organ-system level of organisation.
• They are cylindrical [bilaterally symmetrical], triploblastic, coelomate animals.
• The body is Circulatory system is of open type.
• Respiration takes place through gills.
• Excretory organ is present.
• Sexes are separate. Fertilisation is external. Development is indirect.
• Examples: Balanoglossus and Saccoglossus.

Phylum – Chordata

• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• They are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
• They possess a post anal tail and a closed circulatory system.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
• In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life.
• Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Amphioxus or Lancelet.

All chordates possess the following features:

1. have a notochord
2. have a dorsal nerve cord
3. are triploblastic
4. have paired gill pouches
5. are coelomate.

Vertebrata

• These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.
• The members of subphylum Vertebrata possess notochord during the embryonic period.
• The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
• Thus all vertebrates are chordates but all chordates are not vertebrates.
• Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.
• Vertibrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs.