The volvox might appear to live and function as a simple multicellular organism, but it
is in fact a colony of several thousand single algal cells attached together.
Each individual cell of a volvox colony is very similar in size and structure to Chlamydomonas,
a microscopic single-celled alga that is able to move around and carry out on its own all
the functions of life.
But organisms as small as these are extremely vulnerable.
This one is being engulfed by a much larger protistin, an amoeba.
The individual cells of a volvox colony, however, are better protected from predators by being
attached together.
But for the colony to survive and function as a whole, they have to cooperate in a few
simple ways.
To allow the colony to move, for example, the cells all have to beat their flagella
in unison.
Sponges are very much larger organisms, but each one is really a mass of millions of individual
cells.
There are, however, five or six different types of cell in a sponge, which are all more
dependent on each other than those of a volvox colony, and so the sponge is classified as
a very simple multicellular animal.
The different types of cells that make up a sponge are all loosely arranged in a single-layered
body wall.
Each type of cell performs a particular function.
Some form an outer protective covering called the epidermis.
Some play an important role in strengthening and holding up the wall of the sponge.
Others act as valves, which control the inflow of water that is continually needed by the
sponge for its supply of oxygen and food.
Another group, the collar cells, draw the water into the sponge by beating their flagella
in unison.
Oxygen and food are extracted from the water and then carried to all the other cells by
a fourth group that wander through the jelly-like sponge wall.
These are called amoebocyte cells because they resemble the single-celled amoeba.
Eventually, the water is forced out of an opening at the tip of the sponge.
This process can be demonstrated with the halichondria sponge if some powder is released
from a pipette placed near an opening.
As the powder enters the stream of water being discharged from inside the sponge by the collar
cells, it is quickly swept away.
Just as the amoebocyte cells resemble the amoeba, so the collar cells are similar to
a group of single-celled organisms called the collar flagellates.
It seems possible that sponges evolved originally from protistans similar to these.
But during the course of evolution, the different types of cells that make up a sponge have
all come to rely upon each other to such an extent that sponges are said to represent
the very simplest type of multicellular animals.
The hydra is a member of a group of simple multicellular animals called the silenterates.
There are over 9,000 species, including all the different types of jellyfish, anemones,
and corals.
The hydra, just 2 millimeters long, is another simple multicellular animal made up of many
different types of cells.
But these cells are all much more highly specialized than those of a sponge.
They are arranged in two separate layers.
This is called a diploblastic structure.
The outer layer is known as the ectoderm.
This contains several different types of cells that are needed for defense, movement, and
capturing food.
The inner layer is called the endoderm.
This contains all the cells needed for digestion.
Between the two, there is some jelly-like material called the mesoglera.
This contains a simple nervous system and a number of unspecialized cells.
As the ectoderm and endoderm cells die, some of these unspecialized cells change into the
particular form that is needed to take the place of the dead cells.
All silenterates have the same basic two-layered structure, but the external shape can be either
one of two very distinct forms.
Some, like the hydra, have quite elongated bodies.
This shape is called a polyp and consists of a cylindrical body with a number of tentacles
at one end.
In the middle of the tentacles, it has a mouth.
Behind the mouth and inside the main body, there is a central cavity called the enteron
where food is digested and absorbed.
Other silenterates, including all the different types of jellyfish, have much flatter bodies.
This form is called a medusa because it resembles the mythical Greek figure who wore snakes
instead of hair.
Like the polyp form, it also has a mouth in the middle of its tentacles, which leads to
an enteron.
The flatter medusa shape is more suited for movement, which is achieved by repeatedly
opening and closing its bell like an umbrella.
The muscle cells around the rim of the bell all contract together, forcing the water out
and so pushing the whole organism forward.
But the polyp-shaped species tend to remain in one place, attached by the end of their
bodies to an underwater plant or rock, and use their tentacles to capture the food they
need from the water around them.
In order to coordinate its movements, the hydra has a very simple nerve net contained
in the mesodlia.
This is the most primitive type of nervous system, which gives the animal a basic sensitivity
to chemical stimulation or touch.
When it senses adverse conditions, the nerve net causes the muscle cells in the ectoderm
to contract, producing a rapid shortening of the body.
The nerve net also coordinates the movement of the body and tentacles during the search
for food.
Characteristic of all species of solenterids are a large number of stinging cells covering
each tentacle.
These stinging cells, or nematocysts as they are called, use an elaborate technique for
capturing prey.
Each nematocyst contains a coiled thread, which is released suddenly when a trigger
is touched by the prey.
The threads are discharged with such explosive force that some pierce the body of the prey
to inject a poison.
The sticky threads hold onto the prey, whilst the tentacles move it in towards the mouth.
The anemones are another group of solenterids that are also polyp-shaped.
They remain in one place, but have much shorter tentacles, so they have to rely for their
food on what swims or falls within their reach.
Some types of polyps are colonial.
That is, they live together in groups made up of thousands of individuals.
The most well-known of the colonial polyps are the corals.
As it grows, each coral polyp forms a hard external skeleton made of lime extracted from
the seawater.
And when it dies, its skeleton remains, along with those of millions of other polyps, to
form the massive coral reefs found around the world in warm, shallow oceans.
The Great Barrier Reef, off the northern coast of Australia, has been formed in this way
over millions of years, and is now 2,000 kilometres long and about 80 kilometres wide.
All solenterids can reproduce both sexually and asexually.
Hydra usually reproduce asexually by the process of budding.
A bud appears first as a small outgrowth from the side of an adult.
It starts growing and soon develops a mouth and tentacles, which will enable it to obtain
its own food, although at first its body cavity is still connected to the adult's.
A wall then develops between the two organisms, and eventually the young hydra detaches itself
from the adult to live on its own.
Sexual reproduction, which allows the cross-mixing of different genetic characteristics, occurs
rarely, usually only in autumn.
A female egg is produced in a swelling called an ovary, while sperm cells are formed in
similar but smaller structures called testes.
Most species of hydra are hermaphrodite, which means that they bear both male and female
sex organs.
To avoid sperm fertilising an egg of the same organism, and to promote cross-fertilisation,
the ovary and the testes develop and reach maturity at different times.
When the egg is ripe, it breaks through the covering body wall to expose its surface to
the water.
Within a short time, sperm from the testes of a nearby hydra swim through the water to
surround the egg.
One will then enter the egg and fertilise it.
The fertilised egg, or zygote, will then detach itself completely from the body wall and lie
dormant until conditions are right for it to develop.
In this form, it will survive the adverse conditions of winter that may well kill its
parents and then begin development in the spring, as a new hydra combining the genetic
characteristics of two different hydra.
The planarian worm, between 3 and 4 millimetres in length, is a member of a group of animals
called the flatworms, which are the simplest animals in nature to have a three-layered
or triploblastic structure.
Looking at a cross-section, halfway along its body, the planarian has, like the cylindrates,
a protective outer layer of cells, the ectoderm, and an inner digestion layer, the endoderm,
which forms the lining of the planarian's entorum.
Between the two, there is a third, much thicker layer of cells, called a mesoderm.
Because these cells are not exposed to the environment, like those in the ectoderm, or
adapted for digesting food, like those in the endoderm, they have been able to become
even more specialised for other functions.
Groups of cells in the mesoderm have become joined together to form simple organs, such
as muscles, nerve cords, reproductive organs, and a rudimentary excretory system.
Although still very primitive, these organ systems enable the planarian to perform the
various functions of life more effectively than the simpler two layers of cells in the
cylindrates.
A much more developed nervous system is possible with a three-layered structure.
There is a massive nerve tissue in the head of the planarian, which forms a very simple
brain.
Behind the brain, there are two main nerve cords which run backwards through the planarian's
body, and branching off these are a number of much smaller transverse nerves, as well
as hundreds of minute nerve strands that connect the central nervous system with the outer
surface of the body.
Along with the nerve net of hydra and jellyfish, the nervous system of a flat worm is much
more complex and allows the organism greater control in its response to external stimuli.
Planarians normally live in the dark.
When they detect a source of light, they automatically turn and move away from it.
A planarian senses light with two very primitive eye spots at the front of its body.
There the two lighter patches immediately in front of the main dark body area.
These eye spots cannot see at all in the normal sense.
They can just detect the presence of light and its direction.
The planarian also has a large number of sensory cells distributed all over the body's surface,
which are able to detect touch, temperature, and chemical stimuli.
When these cells detect, by chemical means, a likely source of food, the planarian automatically
turns to move towards it.
It feeds by means of a long tube called a pharynx, which it extends from the middle
of its body.
This tube sucks up microscopic live organisms or pieces of larger dead animals.
The food is drawn into the enteron, where the process of digestion begins.
Like the cylindrates, the planarian has no anus, and so the pharynx is used for both
ingestion and excretion.
Also like cylindrates, the flatworms have no respiratory system, and so oxygen must
diffuse into the organism through its ectoderm or skin.
It is because of this need for a large external surface area in proportion to the internal
volume that the flatworms have developed their distinctive long, flat body shapes.
Other types of worms are much more advanced than flatworms, but they still have the same
three-layered structure, as do, in fact, all species of animal life that have developed
beyond the flatworm stage, from earthworms to humans.
Obviously, in human beings, it has become incredibly complex, and it's now very hard
to distinguish the three cell layers.
But perhaps we can see how they originated in an organism like the flatworm.
Its third layer, or mesoderm, marks a crucial development beyond the rudimentary mesodlear
of the two-layered cylindrate.
And in turn, the cylindrate, with a clearly defined two-layered cell structure, represents
a considerable advance beyond the sponge.
And the sponge, with its loosely arranged, single-layered body wall, containing some
individual cells that bear a close resemblance to single-celled organisms, marks a point
where colonies of individual cells become simple, multicellular animals.