Sierra College Department of Biological Sciences

Discover the Biological Sciences

The introductory information for this site has been contributed as a joint assignment by students in Bio. Sci. 10.

Zoology - Invertebrates

By: Sean Tierney


Invertebrates are animals that do not have backbones. 95% of all animals are invertebrates. The different categories of invertebrates are sponges, cnidarians, flatworms, roundworms, annelids, arthropods, mollusks, and echinoderms.

The areas where more study could appear would definitely be within the species of invertebrates. Here is a brief explanation of each species of invertebrate. Sponges are small plant like creatures that live on the sea floor. The Cnidarians are categorized by having body tissue. A few common Cnidarians are jellies, sea anemones, hydras, and coral animals. Flatworms are quite self explanatory; they are a flat, leaf like shape and range from 2mm to 20m in length. Roundworms differ from flatworms because roundworms have a cylindrical body that is tapered at both ends. There are about 90,000 species of roundworms known, making them the most diverse of all animals. Annelids are worms that have body segmentations; these are areas where the ridges down the worm look as if they had been fused together. The most common type of annelid is the earthworm. Arthropods are segmented animals as well, but what differs them from annelids is that arthropods have special jobs given for each segment (walking, feeding, sensory reception, swimming, and defense). Molluscs are soft bodied animals that have a hard shell to protect them (slugs, squid, and octopuses). The Echinoderm species are known for their spiny exterior surfaces. Common examples of echinoderms are sea stars, sea urchins, and sea cucumbers.

The Arthropod species is a very interesting point of study because they are so numerous; there are about one hundred and fifty million arthropods for every person. The arthropod body is protected by an exoskeleton. This enables the species to keep parts that need to have lots of protection hard as well as making tender areas where bending will regularly occur (such as the joints).

The four different types of arthropods are arachnids, crustaceans, millipedes and centipedes, and insects. Examples of arachnids are scorpions, spiders, ticks, and mites. Arachnids usually have four pairs of legs. The second type of species is the Crustaceans; this species includes crabs, lobsters, crayfish, shrimps, and barnacles. Crustaceans are all nearly aquatic. The third species is the millipede/centipede species. The main personality difference between millipedes and centipedes is that millipedes eat decaying plants and centipedes are terrestrial carnivores. The last species are the insects. Insects outnumber all other forms of life combined. The usually have a three part body, which is the head, thorax, and abdomen. There are also many insects that can undergo metamorphosis.

I found the fact that a giant Australian earthworm can grown to three meters long very interesting.

Reference Used:

Campbell, Reece Simon. Essential Biology. pgs. 348-359.
Further Information:

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