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Characteristics that distinguish animals
The Animalia Kingdom is defined according to characteristics common to all animals: eukaryotic organisms, multicellular, heterotrophic, that get their food by eating nutrients from the environment.
Even within such broad criteria we may find exceptions in the function of various factors, such as the adaptation of organisms to special livelihoods. This is the case, for example, with some endoparasites that have lost their ability to ingest nutrients and obtain them by direct absorption of body fluids from parasitized organisms. All animals begin their development from an egg cell or zygote, which arises from egg fertilization by sperm. Thus, sexual reproduction is always present in the life cycles of animals. This does not mean that asexual reproduction does not happen; it occurs and is very important in some groups.
From the zygote, embryonic development begins, which goes through the phases of morula, blastula and gastrula. There are several types of embryonic development, but just to illustrate, we will now represent all of these phases, from the zygote to the gastrula, considering the easiest pattern for the basic understanding of how they occur.
Some animals develop into a set of cells that do not quite form true tissues, while most reach higher levels of organization than tissues, such as organs and systems. It is thus possible to distinguish two major groups:
- Parazoa (parazoan; para = beside, zoa = animal): represented by Porifera (sponges), in which there is no formation of true tissues.
- Eumetazoa (eumetazoarios; eu = true, metazoário = animal): represented by all other animals that have differentiated tissue.
Among the Eumetazoa two other groups are distinguished: organisms that are no higher than the level of organization superior to tissues, of which cnidaria are part, and organisms that already have organs in defined systems, comprising most of the Eumetazoa.
The branch of biology that studies animals is called Zoology (zoo = animal, logus = study).
it's very common in zoology to talk about animals invertebrates and animals vertebrates.
Invertebrates are all animals that do not have vertebrae and hence spine. Most animals are formed by invertebrates, such as sponges, jellyfish, planarians, worms, worms, insects, crabs, starfish and others.
The term invertebrate has no taxonomic value and does not correspond to groups such as phylum, class, order or others; It is simply a common term applied to all these animals.
Vertebrates correspond to all animals that have vertebrae, such as fish, amphibians, reptiles, birds and mammals.. Vertebrates correspond to a subphylum within the chordate phylum. Among the chordates, there are invertebrate animals, such as amphioxus, which lives buried in the sand in the marine environment.
Symmetry and Locomotion
Animals of simpler organization, such as several sponges, have irregular shapes and are therefore called asymmetrical.
In other animals, we can pass through their bodies several vertical planes of symmetry that pass along the longitudinal central axis (as in the types of sponges that grow to the approximate shape of the vessel, in the cnidarian and most echinoderms, for example); Each plane allows the animal to be separated into equivalent halves. They are called radial symmetrical, generally cylindrical or bell-shaped animals. Most radial symmetrical animals are fixed to the substrate (adult sponges, cnidarian polyps, etc.), or move slowly (jellyfish, stars and sea urchins, etc.).
However, the symmetry predominates in the animal kingdom is the bilateral. Bilateral animals have left and right sides, ventral and dorsal faces and anterior and posterior extremities. The anterior extremity is where the head is located, which contains the nervous command center.
The posterior extremity is the one in which most of the time the anus and breeding holes are located.
In this kind of symmetry there is a sagittal plane which divides the animal into two equivalent halves. In general, bilateral symmetry is related to the lifestyle of "going after" food in a more targeted way.