We now have a general sense of what animals are and how they initially emerged hundreds
of millions of years ago, so now it’s time to get a better sense of the features of animals
and their formation. Though some animals, most notably the sponges,
completely lack true tissues and organs, most animals have unique germ layers that
develop into different tissue types. These differences become apparent in early development.
Remember that animals reproduce sexually, which simply means utilizing sperm and eggs. Yes,
believe it or not, even things like worms and corals and sponges reproduce sexually. So, for
any animal species, reproduction involves a sperm cell fertilizing an egg cell to create a zygote.
This single cell then goes through a process known as cleavage, where it divides repeatedly,
thus converting a single large cell into many smaller cells called blastomeres. During cleavage,
there is no growth, only a subdivision of mass which continues until normally sized
cells form into a cluster called a blastula. At this point, the embryo has gone from a single
cell to a few hundred or even a few thousand. In most animals, the cells of the blastula then begin
to shift and differentiate into cell groups that will become specialized for different functions
within the animal. It is during this cellular differentiation phase, called gastrulation,
where the germ layers become apparent. First, the blastopore folds inward and forms
a gastrula. Now, two cell layers are present. In diploblastic animals, like the Cnidarians,
these two are their only unique germ layers, the ectoderm and the endoderm. These layers
are separated by a primarily noncellular, water-filled tissue layer called the mesoglea,
which we will discuss in more detail later in the series. The ectoderm develops into epidermis,
the outer layer of the body wall. The endoderm, meanwhile, gives rise to the gastrodermis, the
tissues that line the gut cavity. The mesoglea is not considered to be a true cell layer because any
cells found within it arise from the endoderm or the ectoderm. The cells within each tissue layer
are functionally interdependent. The gastrodermis consists of digestive and muscular cells, and the
epidermis contains epithelial and muscular cells. The feeding and general movements of a jellyfish,
for example, are only possible when groups of cells from both tissue layers cooperate.
Now, unlike the Cnidarians, most animals are triploblastic,
meaning they have a third embryological layer sandwiched between the ectoderm and the endoderm
called the mesoderm. This layer gives rise to supportive cells, contractile cells,
and blood cells. Most triploblastic animals have an organ-system level of organization. This means
that tissues are organized to form the nervous, excretory, muscular, and all other body systems,
which we learned about in the context of humans in the anatomy and physiology series.
The mesoderm forms later in gastrulation after the initial blastopore has folded inward,
in what will either form the mouth, in the protostomes, or the anus in the deuterostomes.
These triploblastic animals are further organized into subgroups based on body
cavities. A body cavity is a fluid-filled space where the internal organs are suspended and
separated from the body wall. Some triploblastic animals, like the flatworms, are acoelomates,
meaning they have densely packed cells called parenchyma. These cells are not specialized
for any particular function. Other animals, like the round worms, are pseudocoelomates,
meaning they have a body cavity that is not entirely lined by the mesoderm.
They have no muscular or connective tissues around their digestive tract. Lastly, the majority of
triploblastic animals, everything from earthworms to insects and humans, are considered to be
coelomates. That means they have a “coelom,” a body cavity surrounded by the mesoderm
capable of a multitude of complex functions. All of these features develop during gastrulation
and guide the overall development of the animal. Once this process of gastrulation is complete,
the cells in each germ layer further differentiate in a process known as organogenesis.
Some cells of the ectoderm form the nervous system, others form the outer epithelium of
the body and everything that entails, from the enamel of teeth to the lens of the eye.
Cells in the mesoderm form the circulatory system, the blood, bone marrow, and blood vessels,
while others form the smooth muscle and connective tissue of the digestive tract. The endoderm begins
to form the respiratory tract, the pharynx, and the lining of the gut. All organs and
organ systems can have their cells traced back to this period of differentiation and organization.
Speaking of organization, there are many terms that we must learn in order to be able to describe
all the different types of animals we will be learning about in this series. So let’s move
forward and learn about the biological grades of organization and symmetry in the animal kingdom.