Invertebrates Facts

  • Invertebrates are multi-cellular and all the cells have different responsibilities to keep the animal alive.
  • Invertebrates have no backbone or bony skeleton or cells wall like all other animals.
  • Invertebrates like echinoderms do not have heads.
  • There are millions of invertebrates living in our house in the name of dust mites which are hardly visible to human eyes.
  • Invertebrates under a process called metamorphosis through which they change form as they grow.



Cnidaria is a phylum having habitat in aquatic and marine environments. The feature that separates Cnidaria from other phylum is cnidocytes. These specialized cells are mainly used in capturing the prey.

The body of Cnidaria having a jelly like substance called mesoglea, it is found between the epithelial cells which is about one cell thickness. Their body is composed of two basic forms namely swimming medusae and sessile polyps.

Both these forms are radially symetrical. The mouth of the Cnidarians is covered by tentacles which bear the cnidocytes. The digestion and the respiration can take place with the help of single orifice and the body cavity that is found in both the forms.

Cnidarians are distinguished into the following major groups:

  • Sessile Anthozoa (sea anemones, corals, sea pens)
  • Swimming Scyphozoa (jelly fish)
  • Cubozoa (box jellies)
  • Hydrozoa (a diverse group)

Cnidarians catch their prey ranging from plankton to animals even larger than their size. They can get nutritious food from endosymbiotic algae, parasites, starfish, sea slugs, fish and turtles. Cnidarians are lack of brain and central nervous system.

Instead of those they have nerve nets composed of sensory neurons. Sensory neurons help them in identifying the smells and senses. There is no separate respiratory organ in Cnidarians; the cell layers are helpful in respiration as absorbing the oxygen and expelling the carbon-dioxide into the surrounding water.

In Cnidarians the reproduction can take place through both sexually and asexually. Sexual reproduction consumes a cycle with both polyp and medusa stages.

Feeding can take place in Cnidarians in many ways like predation, absorption of dissolved organic chemicals, and filtration of food particles from the water and getting nutrients from symbiotic algae within their cells. Most of the Cnidarians get their food from the preys but some can also obtain food from corals Hetroxenia and Leptogorgia.

The cnidocytes present in the mouth of the Cnidarians can be categorized into three forms:

  • Nematocysts
  • Spirocysts
  • Ptychocysts

Nematocysts attack the prey by injecting the venom into them; most species of Cnidarians have Nematocysts. Spirocysts not having any venom but it can entangle the prey with the help of small sticky hairs. Only the Anthozoa have this type of cnidocytes.

The last forms of cnidocytes called Ptychocysts are not involved in the prey capturing mechanism. This form is used to building the protective tubes. It is found only in the order Cerianthria, tube anemones.

Features of Cnidarians

  • Cnidarians are not having any definite shape; they are seen in various shapes
  • The internal sac which is used for digestion in Cnidarians is the Gastrovascular cavity
  • Cnidarians body is composed of three layers namely the outer epidermis, the middle mesoglea and the inner gastrodermis
  • The locomotion in the Cnidarians is carried out by the epitheliomuscular cells, which give rise to other cells namely sperm, egg, cnidocytes and mucus-secreting cells
  • The sac like body is seen in two forms namely a vase-shaped polyp and a bell-shaped medusa

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