tailieunhanh - The Complete Aquarium Guide - Part 4

The invertebrates, as their name indicates, have no backbone. Their body is soft, but it is protected on the outside, by a carapace in the case of the crustaceans, or by a shell in mollusks, or it is supported by an internal calcareous skeleton, as in corals | THE MAIN INVERTEBRATE GROUPS The invertebrates as their name indicates have no backbone. Their body is soft but it is protected on the outside by a carapace in the case of the crustaceans or by a shell in mollusks or it is supported by an internal calcareous skeleton as in corals. Although they are considered less evolved animals than the vertebrates - the group to which fish belong - invertebrates sometimes adapt in surprising ways to ensure their survival. The crustaceans for example can walk or swim to look for food or flee an enemy while corals and anemones unfurl to capture microparticles such as plankton in open water or retract to escape their predators. SPONGES Up until the 19th century naturalists hesitated when classifying sponges animal or vegetable It must be admitted however that their field of research was extensive as there are around 10 000 species of sponges. These very old animals were among the first to appear on earth or more exactly in the sea. Freshwater species are rare and are not found in aquariums but some marine species can be kept in captivity. COELENTERATES These animals slightly more evolved than sponges were also classed as vegetables for many years even now the term animal-flowers is used to describe them. The Coelenterates comprise medusas jellyfish - rarely seen in aquariums apart from a few public ones - and the Anthozoa which include anemones and corals some of which are found in aquariums. WORMS Worms are barely evolved soft-bodied animals. They are more common in aquariums as live food than as residents but are never found in freshwater aquariums. A few specific species can be kept in captivity in sea water. They live in a tube and are often sedentary. The coloring of worms can vary enormously but they are usually blue or purple flecked with white and almost always bicolor. MOLLUSKS Their limp body is protected by a shell which has two parts - these are the bivalves - or a single part - as in the case of he gastropods. The .