Introduction to Animal Diversity – (Part One)

This is a introduction to Animal diversity! Animals are super diverse, and even though this is an introduction, it is going to be a long post. So grab a cup of tea, sit back, and enjoy the read! This is part one and there will be a second part posted later.

The first animal fossils found are around 575 million years old. These are called the Ediacaran fauna, named for the place they were discovered, the Edicara Hills in Australia. Animal diversity was not much till the Cambrian explosion, the period around 542-525 million years ago where animal diversity increased dramatically and exponentially.

Early Embryonic Development Stages

The first topic in this talk is the early embryonic development process. It all begins when a sperm cell from the male animal unites with an egg cell from the female animal, and creates a diploid zygote. This process is called fertilization. The zygote experiences repeated mitosis without cell growth in the process called cleavage. Cleavage forms a multi-cellular blastula, which is usually hollow. The blastula undergoes gastrulation which is when layers of embryonic tissues are created which will in time form body parts. A gastrula is created from this process.

Labeled Gastrula

Some animals do not undergo this process, and instead form larva, an immature form of the animal species that is different structurally than the adult, and this undergoes metamorphosis which transforms the larva into an adult. An example includes butterflies.

The tissues shown in the image are called germ layers – layers that form the organs in animal bodies. The ectoderm is the germ layer that covers the embryo and creates the outer skin of an animal and sometimes makes up the central nervous system. The endoderm is the layer at the center, and develops into the inner lining of the digestive tube or tract called the archenteron as well as the liver and lungs in vertebrates, animals with spines. The mesoderm is the third layer that exists in between the other two layers. This forms muscles and other organs in between the outer skin and the digestive tract of an animal. Animals with all three layers are called triploblastic, and animals with only two of the layers are called diploblastic.

Animals develop in two different modes with the early embryonic development: deutersotomic and protostomic.

Protostome vs Deuterostome Development

With protostome development, cleavage is spiral, meaning the dividing cells are diagonal to the y-axis of the embryo. This is determinate cleavage which causes rigid patterns and this determines the fate of each cell.

On the other hand, deuterostomes experience radial cleavage, where cleavage is perpendicular or parallel to the x-axis of the egg. This is indeterminate because eat cell produced by this cleavage keeps the ability to develop into the finished embryo.

The blastopore is the indent that is created in gastrulation forming the archentreron. The blastopore creates an anus first in deuterostomes. Humans are deuterostomes, so that means at one point you were just an anus! 😀 In protostomes the blastopore first creates the mouth of the animal.

The second overarching topic of this post is the body cavity known as the coelom. A body cavity is a fluid filled area that separates the digestive tube from the skin or the wall of the external body. The coelom is this body cavity, and this is present in triploblastic animals. A true coelom forms from the mesoderm. There are three different body plans, sets of traits that define a grade (an organizational grouping on complexity).

Coelom formation is also different in protostomes and deuterostomes. In protostomes, the archenteron forms while solids of the mesoderm split and form a coelomic cavity, and this is called schizocoelous. In deuterostomes the formation is enterocoelous, which is where the mesoderm buds from the archenteron wall and this cavity forms the coelom.


Coelomates have a true coelom. Examples include annelids. They have an ectoderm, then a tissue layer that lines the coelom and suspends organs formed from the mesoderm. At the middle a digestive tract from the endoderm.


Pseudocoelomates include nematodes and have a body cavity that is partly lined by mesoderm tissue. It is fully functional and it is mainly formed from the blastocoal, the fluid filled cavity of the blastula.


Acoelomates include flatworms do not have a body cavity between the outer body skin and the digestive cavity.

The third and final topic is the symmetry of animal bodies. Radial symmetry is when the bodies radiate from the center. Animals that exhibit this include sea anemones.

Radial Symmetry

Bilateral symmetry are in animals that have dorsal or top sides, ventral or bottom sides, right and left sides, a anterior or head, and a posterior or tail. Cephalization is an evolutionary trend that is when bilaterally symmetrical animals have sensory organs in the anterior side with a central nervous system. Animals in this grade include lions, lobsters, humans, lizards, etc.

Bilateral Symmetry

That is it for this introduction to animals part one talk! Tired? Make sure to read part two which is going to distinguish between many phyla and their traits in animals! Until then stay curious and reading! Thank you. 😀

-Written By: Neil D. 4/1/2019


Biology 7th Edition Campbell Reece –

Neil A. Campbell 
Jane B. Reece

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