school energy&environment

How many animals?

The animal kingdom is characterized by the exceptional range of organisms that belong to it. In fact, there are at least 30 million species. You can have an idea of this variety of species by having a look at the different Phyla animals are grouped in.
Among the animals that live in water habitats there are: Porifera, that is sponges, Cnidaria like jellyfish, corals, sea anemones, and hydra. In particular, polyps are able to build limestone support structures that form coral reefs.
Platyhelminthes are flat worms, often hermaphrodites and parasites of the intestine such as the taenia, characterized by a structure of hooks and suckers that hang on the intestine walls in the host organism. Cylinder-shape worms are called nematoda; anellida are earthworms and leeches (important decomposers and parasites).
Molluscs are animals with a soft body because they do not have an internal skeleton, but they simply have an external calcareous skeleton. Some of them are covered in a shell, others in two shells, like clams and mussels. Gastropoda are the most numerous molluscs. They can be aquatic (limpets) or terrestrial (snails and slugs). Cattlefish and octopuses are cephalopoda (sea organisms without shell).
The group of Arthropoda is the most numerous: more than a million terrestrial, aquatic and aerial species belong to it. They are characterized by an external skeleton made up of chitin and proteins, and a body that is subdivided into: head, thorax, abdomen, appendixes similar to “legs”. Moreover their respiratory and nervous systems are well developed. Some examples are: myriapoda like millipede and centipede (terrestrial animals that eat insects by killing them with the poisonous pliers they have on their head); crustacea like crayfish, lobsters and crabs; arachnida like spiders, scorpions, ticks and mites; insects such as bees, butterflies, ants, etc.
Sea organisms like sea urchin, starfish, and holothurians (or sea cucumbers) all belong to Echinodermata phylum.
Finally there are the Chordata, the ones without spine and those with spine (vertebrates like fish, amphibians, reptiles, birds and mammals).