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Invertebrates are the most abundant organisms on earth. They occupy almost all habitats, they can be found crawling, flying, swimming or floating. Invertebrates are the animals without backbone. These animals do not have internal skeleton made of bone. They play a vital role in the earth's ecosystem.
About 99 per cent of the known organisms are invertebrates. Out of the planets estimated 15-30 million species about 90% of the animals are invertebrates. These come in may shapes and sizes and provide services that are vital for our survival.

The most common vertebrates include sponges, annelids, echinoderms, molluscs and arthropods. Arthropods includes insects, crustaceans and arachnids.

The earliest fossil of an invertebrates dates back towards the late Precambriam period, over 600 million years back.


What are Invertebrates?

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Invertebrates are the animals that do not have a backbone or the vertebral column.Animals without the notochord are invertebrates. Most of the animals are invertebrates. The term invertebrates is a prefixed form of a Latin derived word 'Vertebra'. 'Vertebra' means joint in general, specifically it means 'the joint of the spinal column of the vertebrate'. It id coupled with the prefix "in" meaning not or without, which conveys the meaning 'those that lack veterbrae'.

Invertebrates are the most diverse group having about 12 million live species. Most of the animals on earth are invertebrates. They are cold-blooded animals; their body temperature depends on the temperature of the atmosphere.

Characteristics of Invertebrates

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General characteristics of invertebrates are as follows:
  • The main characteristic that separates invertebrates from other organisms is the absence of the spinal column and backbone.
  • They are multicelluar organisms, they completely lack cell walls.
  • They are devoid hard bony endoskeleton. 
  • Due to the lack of complex skeletal systems, some invertebrates tend to be slow and small in nature. 
  • Due to the lack of the backbone and complex nervous system the invertebrates cannot occupy mulitple environments, though they are found in the harshest of the environments. 
  • Invertebrates live all over the world in various habitats.
  • Body is divided into three parts - head, thorax and the abdomen.
  • They do not have lungs for respiration.
  • Respiration is through skin.
  • Some invertebrate groups possess a hard, chitinous exoskeleton. 
  • Most of them have tissues, that are specific organization of cells.
  • Most of them reproduce sexually by the fusion of the male and female gametes. 
  • Few invertebrates like the sponges are sedentary, but most of the organisms are motile. 
  • Most invertebrates are organized with symmetric body organization.
  • They can not make their own food, are heterotrophs.

List of Invertebrates

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Invertebrates are the most diverse organisms present on earth. Almost 95% of the animal populations are of the invertebrates. Based on the International Union regarding Conservation of Nature at the time of 2009 more than 1.3 million invertebrates were identified. Invertebrates make up around 75% of the recognized species on Planet. 

The actual number of invertebrates is unknown, there are several predictions that there may be tens of millions of invertebrates, majority being the insects. New species of invertebrates are being discovered regularly, another worrying fact is that there is insufficient information about these organisms, the invertebrates might be going extinct and the scientists would never know that they even existed. 

Below are a list of invertebraes - 

Crustaceans, Centipedes, Ants, Wasps, Spiders, Locusts, Honey bees, Termites, Cockroach, Grasshoppers, Crickets, Stick insect, Mantis, Crabs, Songes, Star fish, Unio, Leeches, Earthworms, etc.

Invertebrates Classification

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Some invertebrate phyla are:

Phylum Porifera (Sponges)

  • Members of this phylum are commonly known as sponges.
  • Habitat - They are mostly marine, few are found in fresh water.
  • Body symmetry - mostly are assymetrical animals, no definite shape to the body.
  • Level of organization - These are primitive animals, multicellular with cellular grade of organization.
  • Motility - Adult sponges are sessile, that is they need a substratum to attach themselves to a surface and do not move.
  • Mode of Nutrition - Due to the sessile nature, sponges are filter feeders.
  • Digestion - Digestion is intracellular.
  • Skeleton - The body of sponges is supported by a skeleton made of spicules or spongin fibres.
  • Reproduction - Sexes are not separate, they are hermaphrodites. Hermaphroditism - condition where eggs and sperms are produced by the same individual.Sponges reproduces asexually by fragmentation and sexually by formation of gametes.
  • Fertilization - Fertilization is internal.
  • Development - Indirect development, having a larval stage which is morphologically distinct from the adult.
  • Water transport or Canal system - Sponges have water transport system. Water enters through minute pores (ostia) in the body wall into a central cavity known as spongocoel. From the spongocoel water goes out through the osculum. This water system aids in food gathering, respiratory exchange and removal of wastes.
  • Choanocytes - These are collar cells, they line the spongocoel and the canals.
  • Examples: Sycon, Euspongia, Spongilla.

Porifera Sponges

Phylum Cnidaria (Coelentrata) 

  • Habitat - They are aquatic mostly marine animals, they are sessile or free-swimming.
  • Level of organization - They exhibit tissue level of organization.
  • Body wall - They are diploblastic animals, body wall is made of 2 layers, outer ectoderm and inner endoderm.
  • Body symmetry - They are radially symmetrical.
  • Digestive system - They have a central gastro-vascular cavity with a single opening hypostome, which serves as both the mouth and the anus.
  • Digestion is extracellular and intracellular.
  • Specialized cells known as the cnidoblasts or cnidocytes which contain the stinging capsules or nematocytes are present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey.
  • Some cindarians like corals have skeleton composed of calcium carbonate.
  • Cnidarians exhibit two basic body forms called polyp and medusa. The polyp is a sessile form ann cylindrical in shape like the Hydra, Adamsia etc. The medusae form is umbrella shaped and free-swimming forms like Aurelia or jelly-fish.
  • Metagenesis - Cnidarians which exhibit both polyp and medusae form are exhibit alteration of generation, this is known as metagenesis. Poplyps produce medusae asexually and medusae form the polyps sexually.
  • Locomotion - The body contains nerve network that allows movement of tentacles and body.
  • Examples: Aurelia (medusa), Adamsia (polyp).
Coelentrate Jellyfish

Phylum Ctenophora :

  • Ctenophores are commonly known as sea walnuts or comb jellies.
  • Habitat - They are exclusively marine animals.
  • Body symmetry - They are radially symmetrical.
  • Body wall - They are diploblastic organisms.
  • Level of organization - Ctenophores exhibit tissue level of organization.
  • Digestion is both extracellular and intercellular.
  • Locomotion - The body bears eight external rows of ciliated comb plates, these help in locomotion.
  • Bioluminescence is a property of living organism to emit light is well-marked in ctenophores.
  • Reproduction - Sexes are not separate, reproduction is by sexual means only.
  • Fertilization is external with indirect development.
  • Example: Pleurobrachia

Phylum Platyhelminthes

  • They are commonly called as flatworms, they have dorso-ventrally flattened body.
  • Habit - They are mostly endoparasites found in animals and human beings.
  • Habitat - Fresh-water and salt water; terrestrial.
  • Body symmetry - Flatworms are bilaterally symmetrical.
  • Body wall - They are triploblastic animals.
  • Coelomic cavity - They are acoelomate animals
  • Level of organization - Organ level of organization.
  • Digestive system - In flatworms digestive system is incomplete, that is the digestive cavity has a single opening.
  • Parasitic flatworms possess hooks and suckers to hold on to the body of the host. Some forms absorb nutrition directly from the host, through their body surface.
  • Osmoregulation and excretion is carried out by specialized cells cells flame cells.
  • Sexes are not separate.
  • Fertilization is internal and development is indirect, having many larval stages.
  • Some members like Planaria possess high regeneration capacity.
  • Examples: Taenia (tapeworm), Fasciola (liver fluke).
Tape Worm

Phylum Aschelminthes (Nematoda)

  • The body of worms of aschelminthes in cross-section is circular, hence the name round worms.
  • Habitat - They may be free-living, aquatic and terrestrial or parasitic in plants and animals.
  • Level of organization - Round worms have organ-system level of organization.
  • Body symmetry - They are bilaterally symmetrical.
  • Body wall - They are triploblastic animals.
  • Coelomic cavity - They are pseudocoelomate animals.
  • Digestive system - This is the first phylum to have a complete digestive system, with a well developed muscular pharynx.
  • Excretory system - An excretory tube removes body wastes from the body cavity through the excretory pore.
  • They are dioecious - the sexes are separate i.e., males and females are distinct. Often females are longer than the males.
  • Fertilization is internal.
  • Development may be direct - the young ones resemble the adult, or indirect.
  • Examples: Ascaris (round worm), Wuchereria (filaria worm), Ancylostoma (hookworm).
Round Worm

Phylum Annelida (Segmented worms)

  • Habitat - They may be aquatic either marine or fresh water; or terrestrial; free-living and sometimes parasitic.
  • Level of organization - They exhibit organ-system level of body organization and bilateral symmetry.
  • Body wall - They are triploblastic.
  • Coelom - They are coelomate animals.
  • Body is metamerically segmented. The body surface is distinctly marked out into segments of metameres and hence, the phylum name Annelida.
  • Locomotion - They possess longitudinal and circular muscles which help in locomotion.
  • Aquatic annelids posses lateral appendages, parapodia which help in swimming.
  • Circulatory system is closed.
  • Osmoregulation and excretion is by Nephridia.
  • Neural system - It consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
  • Nereis is dioecious, but earthworm and leeches are monoecious.
  • Reproduction is sexual.
  • Examples: Nereis, Pheretima (earthworm), and Hirudinaria (blood sucking leech)
Earthworm Annelida

Phylum Arthropoda

  • It is the largest phylum of the Animalia.
  • It includes insects, spiders, crayfish, etc.
  • Level of organization - They have organ-system level of organization.
  • Body symmetry - They are bilaterally symmetrical.
  • Body wall - triploblastic, segmented. Coelomate animals.
  • The body of arthropods is covered by chitinous exoskeleton.
  • Body is divided into head, thorax and abdomen.
  • Jointed appendages - arthros - jointes, poda - appendages, hence the name is derived from this characteristic.
  • All the arthropods have jointed appendages which give arthropods a wide range of controlled motions.
  • Respiration is through organs like gills, book gills, book lungs or tracheal system.
  • Circulatory system - It is of open type.Sensory organs are present, antennae, eyes (compound and simple), statocysts or balance organs are present.
  • Excretion - It takes place through malphigian tubules.
  • Mostly they are dioceious. Fertilization is usually internal.
  • They are oviparous animals.
  • Development may be direct or indirect.
  • Examples: Honey bee, Silkworm, Lac insect, Mosquitoes, Locust, Crab
Apis Bee

Phylum Mollusca

  • It is the second largest phylum.
  • Habitat - Molluscs are terrestrial or aquatic; they may be marine or fresh water.
  • Level of organization - They have an organ-system level of organization.
  • Body symmetry - Bilaterally symmetrical.
  • Body wall - triploblastic. Coelomate animals.
  • Body is covered by a calcareous shell.
  • Body is unsegmented, they have a distinct head, muscular foot and visceral hump.
  • The radula - Mouth of the molluscs contain tongue-like organ called radula, which has many rows of teeth, which is used to scrape food.
  • Mantle - It is a fold of skin that surrounds the body organs.
  • It is a soft and spongy layer of skin that forms a mantle over the visceral hump.
  • The space between the hump and the mantle is called the mantle cavity.
  • Feather like gills are present in this cavity.
  • These gills have respiratory and excretory in functions. Anterior head region has sensory tentacles.
  • Example: Pila, Octopus, Pearl oyster, Loligo, Sea-hare, Chiton.
Octopus Molluscs

Phylum Echinodermata

  • All the members are marine, live mainly on the ocean floor.
  • These animals have an endoskeleton of calcareous ossicles, and hence the name echinodermata (spiny bodied).
  • Level of organization - Organ-system level of organization.
  • Body symmetry - The adults are radially symmetrical, but the larvae are bilaterally symmetrical.
  • Body wall - Triploblastic. Coelomate animals.
  • Digestive system is complete.
  • The mouth is present on the ventral side and anus on the dorsal side.
  • The most distinctive feature is the water vascular system.
  • This helps in locomotion, capture and transport of food and respiration.
  • Excretory system is absent.
  • Sexes are separate.
  • Reproduction is by sexual means.
  • Fertilization is usually external.
  • Development is indirect with free-swimming larva.
  • Example: Star fish, Sea urchin, Brittle star.
Starfish Echinodermata

Examples of Invertebrates

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Invetebrates include all animals that do not belong the phylum chordata. Common examples include clams, snails, spiders, cockroach, worms, star fish, octopus.

Porifera - Sponges, Sycon (scypha), Spongilla (fresh water sponge) and Euspongia (bath sponge).

Cnidaria - Aurelia, Adamis, Hydra, Sea anemones, Physalia (Portugese man-of-war), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).

Ctenophora Pleurobranchia and Ctenophora.

Platyhelminthes - Taenia (tapeworm), Fasciola (liver fluke).

Aschelminthes - Ascaris (round worm), Wuchereria (filaria worm), Ancylostoma (hookworm).

Annelida - Nereis, Pheretima (earthworm) and Hirudinaria (blood sucking leech).

Arthropoda - Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect); Mosquitoes - Anopheles, Culex and Aedes; Locusta (locust) ; Limulus (king crab).

Mollusca - Chaetopleura (Chiton), Loligo (squid), Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea hare), Dentalium (tusk shell).

Echinodermata - Asterias (star fish), Echinus (sea urchin), Antedon (Sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).
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