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.
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.
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 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.
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 bivalves half-open their shell - formed, as their name suggests, by two valves - to filter water. In this way they absorb oxygen and capture food particles, particularly vegetal plankton. Keeping them in an aquarium does not, therefore, pose any problems.
The gastropods, related to land snails, have a spiraled shell
which varies in shape, according to the species. An organ in the mouth in the form of a grater, the radula, allows them to graze micro-algae on the decor, glass sides, or plants. This cleaning function constitutes their main attraction for aquar-ists. Do not, however, expect them to gobble up all your unwanted algae, as they play a more preventive role to restrict their untimely growth. They can sometimes attack the leaves of plants or eat a surplus of the food intended for fish. Some species are carnivorous.
The crustaceans' bodies are protected by an articulated carapace. The animal abandons its carapace when it becomes too small due to body growth; this phenomenon is called the molt. The crustacean is particularly vulnerable to attacks from predators during the formation of the new carapace. The crustaceans found in aquariums belong to the decapod group, which have five pairs of walking legs, the first of which are used as pincers, with varying degrees of strength. The two pairs of antennae, highly developed in shrimps, play a tactile and sensory role. Crustaceans are carnivorous, and can feed on live or dead prey - they are not difficult to feed in an aquarium.
These possess a symmetry based on five, which is extremely rare in nature, as most animals have a binary symmetry, meaning that, if they are cut down the middle, two identical parts can be observed. This is not possible with echino-derms, because they have to be cut into five sections to obtain identical pieces. Echinoderm means spiny skin: this is highly appropriate in the case of sea urchins, less so in relation to the rough starfish. Generally speaking, echinoderms will not survive for long outside water. They are found only in sea water.
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