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.



Annelids are coelomate animals that are medium size and belong to the class Annelida. The phylum Annelids has more than 9000 worm species. Species in the annelids group can either be aquatic or terrestrial. Though there are more than 9,000 species only a few species are known to man kind. They are Rain, Dew, Earthworms, Ragworms, Lugworms, Tubifex, Red worms, and leeches.

Annelids are very powerful and competent animals which are notable by their ring like external bands and the muscular body wall. The bodies of the annelids are divided into segments. Annelids invaded the earth more than 120 million years ago and can be seen all over the globe.

The main and common characteristic of annelids is that they do not have any limbs. They have well developed internal organs. Annelids have bristles which might be long for some species and short and soft for bristles.

Annelids range from giant earthworms to smallest worms in size. The biggest annelids reported so far is the Michrochaetus rappi which measure about 1.36 m and the smallest annelids is the Chaetogaster annandalai which measures 0.5 mm.

There are many types of worms in the group annelids. They include earthworms, flatworms, and roundworms. The worms can be measure in the range of about an inch to several inches long. The worms like ribbon worms have the ability to grow about 100 ft in length. Some worms are parasites and they are found living inside the humans.

Features of annelids

  • Have elongate and bilateral body which is segmented
  • They have true body cavity
  • They have a circulatory system which is complete
  • The body wall is made up of muscles and they are in circular shape
  • From mouth to the anus they have a continuous gut running with their own musculature
  • Except in leeches other have a bristle like structure which projects from the body

With the help of two muscles annelids can creep and burrow themselves in the soil. One set of muscles help the annelids to expand the body and anchor one part of the body while the other set of muscles help them to contract and push the other end of the body forward. With the help of these two alternating muscles the annelids move inch forward.

Annelids can either be monocious or dioecious. In the life of annelids the larva stage may or may not be present. If the species are the larva stage then they belong to the type trochophore. Some species reproduce through asexually. To learn more about the characteristic of annelids visit the page Characteristic of Annelids

Annelids have an elongated body that is composed into a series of segments. The body part between the head and the tail is segmented. Growth occurs laterally. New segments are formed and in some species the new segments are formed through their life cycle. To learn more about the body and its structure of annelids visit the page Anatomy of Annelids

The phylum annelids can be classified into three major classes. They are Clitellata, Polychaeta, and Aelosomatida. Refer to Classification of Annelids page for more information about the classification.

Some common types of annelids are Bloodworms, Nightcrawlers, leeches, antennae, and other segmented worms.

Recent Videos

Snowball(TM) Rocks The World Science Festival
Very Smart Mathematical Dog

From the Gallery