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What differentiates animals of the plants is the need to eat. From this specificity results a classification of animals according to means and methods used to get food.
According to this classification, species are distinguished in groups with more or less elaborate techniques evolve to feed.
The Elementary Forms
Sponges are cell colonies that uses the simplest way to eat, they filter water to catch their prey. Sponges are the simplest expression of the animal world, no nervous system and even less of a gut, they consist of two cell layer one ectoderm and another endoderm.
Hydra, corals and jellyfish are grouped in the family of polyps. What differentiates a polyp of a sponge is the specialization of these cells and the presence of a gastric cavity. This allows polyps to digest prey larger than those of sponges through the establishment of digestion.
The final basic form of the animal world is the worm. They are characterized by the possibility of moving to find food. Unlike jellyfish, moving offers access to organic matter that are not moving. The gastric cavity became a digestive tract that is the basis of much more varied lifestyle than polyps.
Molluscs have evolved from the structure of the worm, the main specificity of the mollusk from the worm is the appearance of a defense system against different predators. This defense takes the form of limestone slabs acting as armor against external aggression.
Arthropods are an advanced form of worm, the segmentation into same sections of anatomy, presence of legs and an exoskeleton are in their primitive form the millipede.
Insects are arthropods whose body is segmented into three: one is the head, mid abdomen and thorax. Some of these segments are more or less atrophied as a function of evolution.
Fishes are distinguished from mollusks by their ability to swim, their bodies covered with scales. Their complex structures and organs such as gills or fins are specific to this form of animal life.
Tetrapods are animals with four members. Some tetrapods, such as snakes, keep only traces of these members. These species represent the greatest diversity of form and size (from the whale to the mouse) but represent only a small portion of living species on earth.