Animal Kingdom: Invertebrates

Animal Kingdom: Invertebrates

Table of Content

Define Invertebrates

Invertebrates are group of animals without vertebral column. They do not have vertebrae at any stages of their life.

Characteristics of Invertebrates

  • Absence of vertebral column or backbone.
  • They are heterotrophic organisms.
  • They can have radial, bilateral, or spherical in symmetry.
  • They have open respiratory system with spiracles, trachea and tracheoles.
  • Reproduction occurs via partial sexual reproduction. Reproductive cells produce male gamete sperm and female gamete ova.

Basis of classification

Animals are classified based on coelom, body symmetry, patterns of digestive, circulatory or reproductive system etc. Some of the terms related to classification are as follows-

  • Circulatory system can be open or closed. Open circulatory system is a system in which heart and tissues are bathed in blood. Closed circulatory system is a system in which blood is circulated in blood vessels such as artery, veins, or capillaries.
  • Symmetry can be radial or bilateral. If a body can be divided into two equal halves then the symmetry can be radial whereas if only one plane can divide the body into two equal parts, such symmetry is known as Bilateral Symmetry.

  Types of symmetry

Fig. 1. Types of Symmetry

  • Animals are also classified based on the embryonic layers. They can be diploblastic, that is, outer ectoderm and inner endoderm. They can also be triploblastic, that is, outer ectoderm, middle mesoderm, and innermost endoderm.
  • Presence or absence of body cavity is another basis for classification Coelom is a body cavity lined by mesoderm. Animals having coelom are known as coelomate. Animals having scattered mesoderm that surround the coelom, they are known as pseudocoelomates. Animals without coelom are known as acoelomates.
  • Another basis for classification is metamerism. Body divided into segments externally or internally are known as Metamerism.

Classification of Invertebrates


  • The animals of this phylum are known as sponges.
  • They are mostly marine and asymmetric in nature.
  • They have cellular body organization.
  • They have water transport system known as canal system.
  • They have minute pores known as Ostia, through which water enters the body. The cavity in which the water enters is known as Spongocoel. Water exits the body through Osculum.
  • Special cells known as Choanocytes or collar cells are found lining the spongocoel or the water canal.
  • Their skeleton is made up of spicules or spongin.
  • They are hermaphrodite in nature, that is, sexes are not separate.
  • Asexual reproduction occurs by fragmentation. Sexual reproduction occurs by the formation of the gametes.
  • Fertilization is internal with indirect development.
  • For example, Spongilla, Sycon etc.

Examples of phylum Porifera

Fig. 2. Examples of phylum Porifera

Phylum – Coelenterata (Cnidaria)

  • They are mostly marine.
  • They can be free swimming or sessile (attached to the substratum).
  • They have specialized stinging cells known as Cnidoblasts or Cnidocytes. These stinging cells helps in capturing food, defense against the enemies and anchorage.
  • They have tissue level of organization as compared to Porifera which have cellular level of organization.
  • They are diploblastic in nature.
  • They have gastro-vascular cavity with a single opening known as mouth or Hypostome.
  • Digestion can be intracellular or extracellular.
  • They have skeleton composed of Calcium carbonate. For example, Corals.
  • They exist in two basic forms- Polyp and medusa. Polyp is sessile and cylindrical in shape such as Hydra. Medusa is free swimming and umbrella shaped, such as Aurelia.
  • Polyp produce medusa after asexual reproduction whereas medusa form polyp after sexual reproduction. This is known as alternation of generation or Metagenesis.
  • For example, Adamsia, Physalia, Gorgonia, Meandrina etc.

Examples of phylum Cnidaria

Fig. 3. Examples of phylum Cnidaria

Phylum – Ctenophora

  • Commonly they are known as sea walnuts or comb jellies.
  • They are exclusively marine.
  • They are radially symmetrical and diploblastic in nature.
  • Their body possess eight ciliated rows known as comb plates.
  • Digestion can be intracellular or extracellular.
  • They exhibit a property of Bioluminescence (the property to emit light by an organism).
  • Reproduction occurs only by sexual means.
  • Fertilization is internal.
  • Development is indirect.
  • For example, Pleurobrachia and Ctenoplana.

Members of Ctenophora

Fig. 4. Members of Ctenophora

Phylum – Platyhelminthes

  • They are known as flatworms as they are dorso-ventrally flattened.
  • They are mostly endo-parasitic in nature. They are endo-parasitic to human beings.
  • They are bilaterally symmetrical.
  • They are acoelomate and triploblastic.
  • They have organ level of organization.
  • Parasitic forms have suckers and hooks. These helps in absorption of nutrients from the host.
  • Specialized cells known as flame cells helps in excretion and osmoregulation.
  • Sexes are not separate. That is, they are hermaphrodite.
  • Fertilization is internal.
  • Development includes various larval stages.
  • Some have regeneration capacity such as Planaria.
  • For example, Fasciola (Liver fluke), Taenia (Tapeworm).

Fig. 5. (a) Taenia (b) Fasciola

Fig. 5. (a)  Taenia   (b) Fasciola

Phylum – Aschelminthes

  • They are commonly known as roundworms as they are circular in appearance.
  • They can be aquatic, free living, terrestrial.
  • They have organ level of body organization.
  • They are bilaterally symmetrical.
  • They are triploblastic and Pseudocoelomate.
  • Alimentary canal is complete with muscular pharynx.
  • Excretory pore helps in the removal of the waste from the body.
  • Sexes are separate, that is, they are dioecious.
  • Fertilization is internal with direct or indirect development.
  • For example, Wuchereria (Filaria worm), Ascaris (Round Worm), Ancylostoma (Hookworm).

Members of Aschelminthes

Fig. 6. Members of Aschelminthes

Phylum – Annelida

  • They can be aquatic or terrestrial.
  • They exhibit organ level of organization.
  • They are bilaterally symmetrical.
  • They are triploblastic.
  • They are metamerically segmented (body is divided into segments) and coelomates.
  • They have circular and longitudinal muscles. These muscles help in locomotion.
  • Nereis possess parapodia which helps in swimming.
  • Presence of closed circulatory system.
  • They have Nephridia which helps in excretion and osmoregulation.
  • Some have separate sexes such as Nereis but some do not have separate sexes such as Earthworm.
  • Reproduction is sexual.
  • For example, Hirudinaria, Nereis, Pheretima.

Members of Earthworm

Fig. 7. Members of Earthworm

Phylum – Arthropoda

This is the largest phylum of kingdom Animalia.

  • They have organ level of organization.
  • They are bilaterally symmetrical animals.
  • They are segmented, triploblastic animals.
  • They have exoskeleton made up of chitin.
  • Their body is divided into head, thorax, and abdomen.
  • They have jointed appendages.
  • There are different respiratory organs found such as gills, book gills, book lungs or

tracheal system.

  • Open circulator system. Instead of blood, hemolymph is present.
  • Antenna and sensory organs are found for sensing. Statocysts are for balancing.
  • Malpighian tubules are meant for excretion.
  • Fertilization is internal.
  • Animals are oviparous in nature.
  • Development can be direct or indirect.
  • For example, Locusta (Locust), Anopheles, Culex and Aedes.

Examples of Arthropoda

Fig. 8. Examples of Arthropoda

Phylum – Mollusca

  • They can be aquatic or terrestrial.
  • They have organ level of organization.
  • They are bilaterally symmetrical.
  • They are triploblastic animals.
  • They have unsegmented body.
  • Their body is divided into head, muscular foot, and visceral hump.
  • The space between hump and mantle is known as mantle cavity. Mantle cavity is a region where feather like gills are present.
  • The head region contains tentacles which are sensory in nature.
  • Mouth contains radula which are meant for feeding.
  • They have separate sexes.
  • They have indirect development and oviparous in nature.
  • For example, Loligo (Squid), Pinctada (Pearl oyster), Chaetopleura (Chiton), Pila (Apple snail), etc.

Members of Mollusca

Fig. 9. Members of Mollusca

Phylum – Echinodermata

  • They have calcareous endoskeleton.
  • They have exclusively marine.
  • They have organ level of organization.
  • Adult animals are radially symmetrical whereas larvae are bilaterally symmetrical.
  • They have water vascular system which functions during locomotion, transport of food, food capturing etc.
  • No excretory organ is present.
  • They have separate sexes.
  • Reproduction is sexual and external.
  • Development is indirect.
  • For example, Cucumaria (Sea cucumber), Asterias (Star fish), Echinus (Sea urchin), etc.

Members of Echinodermata

Fig. 10. Members of Echinodermata



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Animal Kingdom: Invertebrates

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