What is the evolution of animals and primary body plans?

Introduction:

The evolution of animals started in the ocean almost  600 million years ago with the formation of small creatures that likely do not resemble any living organism of today’s animal species. Since then, animals are continuously developing into highly various and diverse kingdoms. Although more than one million types of species of animals have been identified and scientists are constantly discovering more species by exploring various ecosystems around the whole world. Many kinds of species of both animals and plants had also been extant. The number of extant species is estimated between 3 to  30 million.

The question is raised here, what is an animal?

 While we can easily identify cats, dogs, birds, fishes, spiders, crustaceans, and worms as animals, other organisms, e.g corals and sponges, are not as easy to classify.  All animals vary in complexity to a great extent —from sponges to grass-hoppers to heavy-weighted chimpanzees and most diversified and complex human beings —and scientists have gone through with the difficult task of classifying them into a proper “organisms system”. They must identify traits and characteristics that are common in all animals as well as traits and characteristics which can be used to differentiate among all groups of animals. The animal classification system represents animals on the basis of their evolutionary history, anatomical, and morphological characteristics, embryological and developmental traits, and their genetic makeup. This classification system is continuously developing with the addition of new information about different species. Understanding the species and classifying the great variety of living species also help us better understand how to conserve the animals on plants, because with the extinction of a species we also lose  the diverse genetic makeup of that specific species.

Step-by-step evolution of animals :

Animals evolved in a large span of time from soft-bodied "Cephalo-chordates" to complex mammals.

• Cephalochordata

• Urochordata 

• Myxini 

• Petromyzontidae

• Chondrichthyes

• Actinopterygii

• Actinidia

• Dypnoii

• Amphibia

• Reptilia

• Mammalia 

The evolution of animals, and all living organisms, is a complex procedure that took billions of years. We can say that modern animals have evolved from different lineages of simplistic aquatic species beginning almost  2.5 billion years ago.

The oldest known ancestors of animals are the “metazoans”, but we don't know much about them except that their shape was very identical to worms. Then, came the “Ediacaran animals” which were a mixture of jellyfish with tougher outer walls. At last, during the “Cambrian period”, many of the ancestors of modern animals like insects, jellyfish, aquatic bivalves, and many more animal forms emerged. Once the evolution of fish started becoming more complex, amphibians allowed a metamorphosis between water and land. Mammal-like reptiles formed around the same time that “dinosaurs” came into presence. The removal of dinosaurs allowed many different animal groups to grow such as mammals and vertebrates such as fish.

 
Body Plan of Animals(Symmetry)

The body plan of animals follows the following types of symmetries:

1. Asymmetry

2. Radial symmetry

3. Bilateral symmetry

Asymmetrical Animals

Asymmetrical animals are animals with no body pattern or symmetry for example sponge is an asymmetrical animal.

 Radial symmetry

Animals having radial symmetry, have an up-and-down body orientation: any plane cut along its longitudinal axis through the organism makes equal halves, without the limitation of the left or right side. This plan is seen mostly in aquatic animals, specifically organisms that bind themselves to some base, like a rock or a boat, and get their food, from the surrounding water as it flows around the organism.

Radial symmetry is found in the jellyfish, sea anemones, and corals as well as in sea urchins, brittle stars, and sea stars.

Bilateral symmetry

Animals having bilateral symmetry have left and right side mirror images of each other when they are cut from the center.  They also have an upper and lower component to them, but a plane cut from front to back separates the animal into definite right and left sides .

Insects, fishes, amphibians, reptiles, birds, mammals, and most crustaceans have bilateral symmetry.

Animals with bilateral symmetry that live in water have a fusi-form body shape: this is a tubular-shaped body that is tapered at both ends. This shape helps to decrease the drag force of the water on the body when they move through the water and allows the animal to swim swiftly at a high speed.