Animals Without a Backbone – Part 1

Around us, we see many kinds of animals. Some are large, like a cow or a dog, and some are so small that we need a microscope to see them. Some animals have a backbone, and some do not. Animals whose bodies are made of only one cell are called unicellular, and animals whose bodies are made of many cells are called multicellular.

Stop and look at the word invertebrate for a moment. It defines an enormous number of animals by something they do not have. That is like sorting every person in the world into two piles labelled people from my village and people not from my village. It tells you almost nothing about the second pile, and the second pile is nearly everybody.

That is exactly the situation here. Out of every hundred kinds of animal known to science, roughly ninety seven are invertebrates. Insects alone outnumber every animal with a backbone many times over. So the animals in this chapter are not the odd exceptions. They are the ordinary case, and cats, snakes, frogs, birds, and people are the small unusual group at the edge. We named the majority after the minority simply because we happen to belong to the minority.

Five names in a row are hard to hold on to. They become far easier once you notice that they are not really a list at all. They are a sequence, and one simple question runs through the whole of it: how does food get in, and how does waste get out?

Read down that last column and the five groups stop being names to memorise. Each one is a body that can do a little more than the one above it, and the biggest single jump in the whole table is between the fourth row and the fifth. Getting a second opening changes everything.

A unicellular animal has a body made of just a single cell. These animals are usually found in water, and some live inside the bodies of other animals as parasites. They move using fake feet called pseudopodia, a whip-like tail called a flagellum, or tiny hairs called cilia. Since their whole body is one cell, they carry out every life activity, such as digestion and reproduction, inside that same cell. The group of unicellular animals is called Protozoa. Amoeba, Paramecium, and Euglena are examples of this group.

You already know why a protozoan can never grow large. From the topic on cells: everything has to come in through the surface, and as a cell gets bigger its inside outgrows its surface, so the middle would starve. A one-celled animal is therefore stuck at a size you need a microscope to see. Every other group in this chapter got past that limit by the same trick, which is being made of many cells instead of one.

Porous animals, commonly called sponges, have a body full of tiny holes called ostia through which water and food enter, and a larger opening at the top called the osculum through which waste water leaves. A sponge stays fixed to one spot, usually underwater, and does not move around. If part of a sponge's body breaks off, it can regrow into a new sponge, this power to regrow lost parts is called regeneration. The group of porous animals is called Porifera. Sponge, Sycon, and Spongilla are examples of this group.

A sponge is a strange kind of animal, because it has many cells and yet no organs at all. There is no mouth, no stomach, no heart, and no nerves. Each cell more or less looks after itself, and the whole body is closer to a colony of cells sharing an address than to an animal with parts. That is why a broken piece can grow into a whole new sponge, which no cat or bird could ever do. There is nothing to be broken, because there was nothing organised in the first place.

Hollow-bodied animals have a body shaped like a hollow tube, with thin, thread-like arms called tentacles around the mouth. These animals use one single opening as both the mouth for taking in food and the way to push out waste, and they use their tentacles to catch food and pull it towards the mouth. The group of hollow-bodied animals is called Coelenterata. Hydra and coral are examples of this group.

One opening for everything sounds harmless until you think about what it means in practice. The animal has to swallow, digest, and then push the leftovers back out of the same hole before it can take anything else in. Eating and clearing up cannot happen at once. Compare that with your own body, where a mouthful you swallowed an hour ago, a mouthful from ten minutes ago, and one you are chewing right now are all being dealt with at the same time in different places along one long tube. That is only possible because your tube has two ends.

Flat animals have a body that is flat, wide, or shaped like a ribbon. Most flat animals are parasites, living inside the body of another animal. Their mouth works as a sucker, through which they suck food from their host. Most flat animals have both male and female reproductive parts in the same body, though usually only one works at a time, this is called being hermaphrodite. The group of flat animals is called Platyhelminthes. Liver fluke, tapeworm, and Planaria are examples of this group.

Why flat, though? The answer is the idea that keeps coming back through this whole unit. These animals have no blood system and no heart, so oxygen cannot be pumped anywhere. It has to seep in through the skin and reach every cell by itself, and it can only travel a very short distance that way. A thick body would have a middle that oxygen never reached. Being flat means no part of the animal is ever far from the outside, however long it grows. A tapeworm can be metres long and still work, because it is only ever a fraction of a millimetre thick.

Count how many times that has now come up. A cell stays small so its surface can serve its inside. A root grows thousands of hairs. A leaf spreads into a thin sheet. A flatworm flattens itself. Four completely unrelated living things, all shaped by the same unavoidable fact about surfaces.

These animals have a long, round, tube-shaped body, but unlike an earthworm, their body is not divided into segments. Their body is covered by a tough outer skin called a cuticle. Most of them are parasites, though some live freely in water. They have separate male and female animals, and unlike flat animals, they have a complete digestive tube with both a mouth and an anus. The group of unsegmented cylindrical animals is called Nematoda. Ascaris, the parasitic roundworm found in the human intestine, and hookworm are examples of this group.

The complete gut is the important part of that paragraph, and it deserves more than one line. With two openings, food only ever travels in one direction, which turns the gut into something like an assembly line in a workshop. Each section can specialise: one part softens the food, the next takes the goodness out of it, the last removes the water. None of that is possible in a bag with one opening, where everything has to happen in the same space at the same time. That is why almost every animal you can name from your own life, including you, has two openings and not one.