What are Invertebrates?

Invertebrates are animals without a backbone. Invertebrates make up the vast majority of all animals, especially insects, which are the most diverse and populous group. Invertebrates are classified into 8 different main phylum: porifera(Sponges), cnidaria(cnidarians), platyhelminthes(flatworms), nematoda(roundworms), annelida(segmented worms), mollusca(mollusks), arthropoda(arthropods), and echinodermata(echinoderms).


Sponges are sessile animals, which means they do not move around. Sponges catch microscopic food by filtering them out of the water. Sponges reproduce both sexually and asexually, since they are hermaphrodites. This means that they have both male and female parts(like flowers).


Cnidarians are hollow animals that have two cell layers. They have tentacles that shoot out stinging cells surrounding their mouth. They use their stinging tentacles to catch their prey. Cnidarians include jellyfish, hydras, sea anemones, and corals.


Flatworms are long, flat worms that are bilaterally symmetrical, which means that one half of their body is identical to the other half. Flatworms have a cavity with one opening for a mouth, no anus(where waste is released), and no circulatory or respiratory system. They do, however, have a digestive system to digest food.

Most flatworms are parasites, like tapeworms. Tapeworms are a type of parasitic flatworm which eats food inside an organism’s intestines. Tapeworms grow new segments, and can grow to be extremely long. To reproduce, tapeworms grow a segment filled with eggs. The segment breaks off, and gets released with the host organism’s waste. The eggs can then enter a new host organism where they live and eat there.


Roundworms are tube-shaped worms with two layers. Roundworms are more complicated than tapeworms, as they have a mouth and anus. Roundworms are also parasites and inhabit host organisms like dogs to survive.


Annelids are also called segmented worms. They are worms with segments, circulatory systems, a mouth, and an anus to release waste. Earthworms and leeches are two types of segmented worms.


Mollusks are soft-bodied organisms which usually have a shell. Examples of mollusks include snail, octopuses, and scallops. Mollusks are surrounded by a layer of tissue called a mantle. The mantle make the shell for shelled mollusks.


Arthropods are animals with appendages like claws, legs, or antennae. They have an exoskeleton, which is a hard outer covering. The exoskeleton does not grow with the arthropod, so the arthropod must shed its exoskeleton and grow a new one. This process is called molting. There are 4 groups of arthropods: insects, myriapods, arachnids, and crustaceans.


Echinoderms are animals with spiny skin and radial symmetry, which means all their body parts are symmetrical and are arranged evenly around the center. Echinoderms do not have heads or brains. Example of echinoderms are starfish, sand dollars, sea urchins, etc.