Included in this group are living invertebrates that are not contained within the orders of insects and spiders. The include both land and marine animals, including crustaceans, mollusks, echinoderms (star fish and sea urchins), and annelids(worms). Many are extremely primitive, both in physical make-up and life-style, but others are quite sophisticated, such as the octopus.
Like insects and spiders, crustaceans are arthropods. There are about 35,000 species. Most of them breath through gills and either in water or in an extremely humid environment. Mollusks have more than three times the number of species, 125,000, showing hugely varied characteristics. They have soft bodies, in which the back is covered with a membrane known as the mantel, which sometimes forms a calcareous shell. They are found in all environments from the bottom of oceans to the top of mountains.
There are nearly 4,000 species of echinoderms, a name that gives a clue to one of the group's characteristics as it means " spiny skin." The spines, however, vary greatly from the long prickles of a sea urchin to the almost unnoticeable eruptions on the skin of a sea cucumber. There are about 20,000 separate species of annelids. Annelids fall into three phyla, all of which are characterized by an elongated, soft, legless body with a head and a tail.
Primitive Animal Fact For Today