OK, so now we have gone from a single cell to a microscopic animal that is a ball of cells that specialise in doing
different things. Now we are going to make a much more complex animal!

In the diagram below, we start with such an animal shown as the blue ovoid with two triangular appendages
(maybe paddles for swimming). Now, many such animals reproduce by binary fission - that is they split into two
(usually crosswise, a process called transverse fission) as shown in the left column. Now, what happens if the
individuals fail to separate, well we get a chain of individuals as shown on the right. Some marine animals do
just this, they divide incompletely to form a
chain of individuals, which may eventually separate. If, however,
the division never completes because the extra individuals do not develop fully, then what do we have ...
Cephalisation
Metamerism
We have a creature like the one on the left below - it is a segmented worm! (Note there is a competing theory
that explains our single animal splitting itself up without going through a failed stage of division, but the end
result is exactly the same!).

When I say 'segmented' I means what is called
metamerically segmented, a phenomenon called
metamerism. In a metamerically segmented animal, not only is it segmented on the outside, but it is also
segmented inside. This means that each segment has its own little brain, its own reproductive organs and its
own excretory organs, etc. Sometimes these structures may fuse together, so the creature may have one pair
of very large gonads again, for example. Many worms that live in the sea, like the ragworms that you find on the
beach, and the earthworms in your garden are metamerically segmented. Each segment of the earthworm has
its own little 'brain', called a ganglion. This is why an earthworm can easily grow back a few segments if they get
severed by your spade. OK, so how do we go from a worm to an insect or a human being?
The segmented worm on the right above has done something slightly odd. Its three front-most segments have
failed to develop fully, they are short and fused together more tightly than the rest. Now this is not a bad thing,
their three little brains have fused into one big brain, so now the animal knows which bit is the boss and which is
its front end! The first three pairs of limbs (paddles or legs) are reduced and not much use for walking, but they
would make excellent sensory structures or 'feelers'. Indeed, a large brain is needed at the front end to process
all the sensory data. This process whereby neighbouring segments fuse into one mega-segment is called
tagmatisation and the mega-segment is called a tagma. In this case the tagma has formed a head, a process
called
cephalisation. A similar process may occur at the rear end, with the back legs becoming long sensory
processes and the rear segments specialising in reproduction - again we have division of labour. This is shown
below:
Wow you say! Now we have an animal that looks like a centipede! Now group some more segments together to
form a head, thorax and abdomen and lose a few legs and you get an insect! Now what do flies grow from?
They grow from worm-like maggots with many segments. This is quite typical that an organism appears to
reiterate its evolutionary stages as it grows. After all you were once a single cell, then a ball of cells, then a tube
of cells ...

their own little ganglion computer (actually a pair because you are also duplicated into a left half and a right
half) that may have been a brain of the individual who developed into that segment! (I say 'may', because result
process certainly has all the intermediate stages still to be found in nature, so they are real processes!). You
humans could also be derived from an individual that duplicated its own parts without ever dividing, but the end
process certainly has all the intermediate stages still to be found in nature, so they are real processes!). You
don't have gonads in your head (well, it has been claimed of certain people!).
have ribs, your muscles are segmented, etc. Not all your organs are so segmented of course, because
segments undergo a variable degree of fusion and some segments have lost certain bits - for example, you
don't have gonads in your head (well, it has been claimed of certain people!).

So there you have it, you are a composite organism (part bacteria!) and you are also a modular organism,
made up of modules (segments) that are made up of colonies of smaller modules called cells, and all this time
you considered yourself an individual!

Return to Nature Tech
Building Animal Bodies - Modules & Segmentation
Bauplans - Body Layers

the tissues in the adult animal develop either from two cell layers in th early embryo (the diploblastic
condition
found in diploblasts) or from three principle cell layers (the triploblastic condition found in
triploblasts). In diploblasts, such as jellyfish, the outer cell layer is the ectoderm, which forms the outer body
wall, and the inner layer is the
endoderm, which lines the gastric cavity.
Above: the diploblastic condition in coelenterates - the outer ectoderm in blue, the inner endoderm in yellow
and the jelly-like mesogloea between them in cyan. The mesogloea may be largely noncellular, consisting of
protein fibres embedded in a fluid matrix to form a gel, or it may be infiltrated by cells from the other layers to
form various muscles and other tissues. The early embryo has only two cell layers, the adult may have two or
more cell layers, but all derived from the two embryonmic layers.
Above: the triploblastic condition as found, for example, in flatworms. As with the diploblast we have the outer
ectoderm (blue), inner endoderm (yellow) and in addition a middle cell layer, the mesoderm (magenta). The
ectoderm forms the body wall, the endoderm the gut lining and the mesoderm the viscera and muscles
inbetween the body wall and the gut. There is no body cavity, so this condition is
acoelomate. Again, in the
adult there are many cell layers, but all derived from these three layers in the embryo.
Above: the triploblastic condition as found in vertebrates, molluscs, echinoderms, earthworms and many other
animals. As with the acoelomate triploblastic condition, there is an outer ectoderm, inner endoderm and middle
mesoderm, but the mesoderm splits early on to form a double layer separated by a body cavity called the
coelom. Mesenteries (mesenteric ligaments) connect the two layers together and also attach to and
surround many of the internal organs, supporting them. The coelom is generally fluid-filled in aquatic animals
and some terrestrial ones where it can function as a hydrostatic skeleton, as in earthworms. In mammals, the
coelom forms the
abdominal cavity, the pleural cavity around the lungs and the pericardial cavity around
the heart. It is generally ciliated to circulate the fluid, which in mammals amounts to a small amount of fluid
circulated as a lubricating sheet which lubricates movements of the internal organs. The lining of the
abdominal cavity is the
peritoneum which circulates peritoneal fluid.
This article last updated: 27/12/2014