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Classification of Invertebrates & General Characteristics

Classification of Invertebrates & General Characteristics


Animals are classified on the basis of some fundamental features like arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive organs.

Levels of Organization

It exhibits consideration variation among animals. Following are the major types:

  1. Cellular level- Cells Are arranged as loose cell aggregates. Individual cells are capable of performing specific functions.

Eg- Sponges

  1. Tissue level- cells performing similar function are aggregated to form tissues.

Eg- Coelentrates

  1. Organ level- Tissues are grouped together to form organs, each specialised for a particular function.

E.g. Platyhelminthes and other higher phylas.

  1. Organ system level- A group of organs working together as a larger unit Is called as organ system. Organ system exist in various patterns:
    • Digestive system
    1. Incomplete- Digestive system has only single opening. It acts as both mouth and anus.
    2. Complete- The digestive system has two separate openings at each end, the mouth and anus.

E.g.- Man

    • Circulatory system
  1. Open type- Blood is pumped out of the heart, and tissues are bathed into it. E.g.- Arthropods and Molluscs
  2. Closed type- Blood Close inside blood vessels of varying diameters (arteries, veins, capillaries) without coming in direct contact with body cells.

E.g.- In many invertebrates and all vertebrates.


Arrangement of body parts on the main median axis of the body. Animal exhibit three types of symmetry:

  1. Asymmetry- Body cannot be divided into two equal halves through any plane passing through centre.

E.g.- Sponges

  1. Radial symmetry- Body Can be cut into two identical halves through any plane which pass through the central axis.

Eg- Coelentrates, Ctenophora, Echinoderms

  1. Bilateral Symmetry- Body can be divided into two identical halves only through a single plane passing through central axis.

Eg-Annelids, Arthropods etc.

Diploblastic and Triploblastic Organisation

This Classification is based on number of embryonic cell layers and divided into two types.

  1. Diploblastic Organisation- Cells are arranged into two embryonic layers i.e outer ectoderm and inner endoderm and Middle undifferentiated mesoglea.

Eg- Sponges and Coelentrates

  1. Triploblastic Organisation- Three embryonic layers are presente outer ectoderm, inner endoderm and middle mesoderm. All tissues and organs are formed from these layers.

Eg- Platyhelminthes to Chordata


The fluid filled cavity in between the body wall and the digestive tract of animals is called the coelom. The coelom is lined by mesodermal epithelium On either side. Animals are divided into three groups on the basis of nature of coelom:

  1. Acoelomates- Body cavity is absent.

Eg- Platyhelminthes

  1. Pseudocoelomates- Body cavity is not line by mesodermal instead, mesoderm is present as scattered pouches In between the ectoderm and mesoderm.

Eg- Aschelminthes

  1. Coelomates- Animals with true body cavity i.e cavity lined by

Eg- Annelids to Chordata


In some animals the entire length of body is transversally e divided into a number of ring like parts called segments. This phenomenon is called Metamerism.

Eg- Earthworm


Notochord is meso-dermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.

Based on the absence of presence of notochords in animals are of two types:

  1. Chordates- Animals with notochord.

Eg- Fishes, Birds, mammals etc.

  1. Non- chordates- Animals without notochord.

Eg- Porifera to Echinodermata


The important characteristic features of different phyla are described as follows:

  1. Phylum- PORIFERA
  • Members of this phylum are known as
  • They are generally marine and mostly asymmetrical.
  • They are primitive multicellular animals.
  • Cellular level of organization is present.
  • Sponges have a water canal system.
  • The pathway of water is helpful in food gathering, respiratory exchange and removal of wastes.
  • Digestion is intracellular.
  • 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.
  • Eg. Sycon, Euspongia, Spongilla
  1. Phylum- COELENTRATA (Cnidaria)
  • They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical and acoelomate animals.
  • They contain stinging capsules or nematocysts called cnidoblasts or cnidocystes, present on tentacles and the body.
  • Cnidoblasts are used for anchorage, defense and for the capture of prey.
  • They have central gastro-vascular cavity with a single opening, mouth or hypostome.
  • They have tissue level of organisation.
  • Digestion is extracellular and intracellular.
  • They exhibit two basic body forms called poly and medusa.
  • Cnidarians which exist in both forms exhibits alternation of generation.
  • Eg. Physalia, Adamsia, Pennatula
  1. Phylum- CTENOPHORA
  • Ctenophores, commonly known as Sea Walnuts or comb jellies.
  • They are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organization.
  • The body bears eight external rows of ciliated comb plates, which help in locomotion.
  • Digestion is both extracellular and intracellular.
  • Bioluminescence (property of living organisms to emit light) is well-marked in ctenophores.
  • Sexes are not separate.
  • Reproduction takes place only by sexual means.
  • Fertilization is external with indirect development.
  • Eg. Pleurobrachia and Ctenoplana
  • They have dorso-ventrally flattened body, hence called
  • They are mostly endoparasite found in animals including human beings.
  • They are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation.
  • Hooks and suckers are present in the parasitic forms.
  • Specialized cells called as flame cells helps in osmoregulation and excretion.
  • Sexes are not separate.
  • Fertilization in internal and development is through many larval stages.
  • Some members like Planaria possess high regeneration capacity.
  • Eg. Taenia, Fasciola
  • The body of aschelminthes is circular in cross-section hence called as roundworm.
  • They possess organ-system level of body organisation.
  • They are bilaterally symmetrical, triploblastic and pseudocoelomate.
  • Alimentary canal is complete with a well-developed muscular pharynx.
  • Sexes are separate (dioecious), females are often longer than males.
  • Fertilisation is internal and development may be direct or indirect.
  • Eg. Ascaris, Wuchereria, Ancylostoma
  1. Phylum- ANNELIDA
  • They may be aquatic or terrestrial; free living and sometimes parasitic.
  • They exhibit organ system level of body organisation and bilaterally symmetrical.
  • They are triploblastic, metamerically segmented and coelomate animals.
  • They possess longitudinal and circular muscles which help in locomotion.
  • A closed circulatory system is present.
  • Nephridia help in osmoregulation and excretion.
  • Some annelids are dioecious while some are monoecious.
  • Reproduction in sexual.
  • Eg. Nereis, Earthworm, leeches.
  1. Phylum- ARTHROPODA
  • This is the largest phylum of Animalia which includes insects.
  • 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.
  • They have joined appendages.
  • Respiratory organs are gills, book lungs or tracheal system.
  • Circulatory system is of open type.
  • Sensory organs are present.
  • They are mostly dioecious. Fertilization is usually internal.
  • They are mostly oviparous.
  • Eg. Honeybee, mosquitoes, silkworm
  1. Phylum- MOLLUSCA
  • This is the second largest phylum of animal kingdom.
  • They are organ system level of organisation.
  • They are bilaterally symmetrical, triploblastic and coelomate animals.
  • Body is covered with calcareous shell and is unsegmented.
  • Gills are present as respiratory and excretory organs.
  • 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.
  • Eg. Pila, octopus, squid
  • These animals have an endoskeleton of calcareous ossicles and hence the name Echinodermata (spiny bodied).
  • They are exclusively 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.
  • Water vascular system is present which helps in locomotion, capture of food and transport of it and in respiration.
  • Excretory system is absent.
  • Sexes are separate. Fertilization is usually external.
  • Development is indirect with free-swimming larvae.
  • Eg. Starfish, Sea urchin etc.

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