1.b.6 – What are the characteristics of animals?

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December 17, 2009 — Riccardo Sabellotti - Giacinto Sabellotti

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What are the characteristics of animals?

The multicellular bodies are divided into fungi, plants and animals; these latter are those who, for living, must feed on other living beings and are generally capable of moving. It is estimated that the appearance of the first animals dates back to around 700 million years ago and that all derive from a common ancestor; this ancestor should have been a complex colony of protists, perhaps similar to the current sponges, whose cells still enjoyed considerable autonomy. Even today the embryos of all animals, including humans, begin their development with a structure of this type, then start the process of differentiation and specialization of cells, that are divided into two or three layers: internal, external and intermediate, from which then derive all our tissues.
The construction of our body seems to retrace its evolutionary history; in fact the comparative anatomy, the fossil remains and genetic comparisons, show us that most known animal descended from a common ancestor, which body was divided into three main layers of cells: an outer protective layer, which in subsequent forms gives rise to skin, the sense organs and nervous system, an internal layer with digestive functions that then gives rise to all organs of the digestive system, and an intermediate layer from which will have origin bones, muscles, blood and the genital apparatus. The other animals descended from another branch, even more ancient, whose body structure was based on only two initial layers.
This ancient ancestor gave rise to many large families such as crustaceans, mollusks, insects, starfish and fish.

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