Animals Without a Backbone – Part 2

These animals have a long, round body that is divided into many small segments, almost like a row of rings joined together. Their skin is soft and moist, and they breathe through this skin. They are usually found in water or damp soil. Most of them have both male and female parts in one body, similar to flat animals. An earthworm and a leech move using tiny bristles called setae or a sticky sucker at the end of their body. The group of segmented cylindrical animals is called Annelida. Earthworm, leech, and Nereis are examples of this group.

Segments look like a small detail and they are one of the most important ideas in the whole animal kingdom. Each segment carries its own set of muscles, so an earthworm can squeeze one part of itself while stretching another, and it moves by sending that squeeze along its body like a wave. A body made of one solid piece could never do that. But the bigger point is what segments make possible later. Once a body is built from repeating units, a few of those units can be changed into something special, a head at one end, jaws, legs, wings, without redesigning the whole animal. Every group that came after this one is built on that trick.

Earthworms also deserve better than their reputation. As one moves through soil it swallows the soil, digests what it can, and pushes the rest out behind it, leaving tunnels that let air and water reach plant roots and returning waste to the earth in a form plants can use. A field with plenty of earthworms drains better, holds more air, and grows more. For a farmer they are not a pest at all. They are free labour working every night.

This is the largest group of animals on Earth. Their legs are made of segments joined together, like the parts of a robot arm, and most have three or more pairs of legs and often wings too. Their body is covered by a hard outer shell called an exoskeleton, instead of a skeleton inside their body. Most of them have compound eyes, made of many tiny lenses, which let them see in almost every direction without turning their head. The group of jointed-leg animals is called Arthropoda. Butterfly, cockroach, prawn, and mosquito are examples of this group.

An exoskeleton is superb protection and it has one serious flaw: it is hard, so it cannot stretch. A growing insect eventually fills its shell completely and can get no bigger inside it. The only way forward is to split the old shell open, crawl out, and wait while a new, larger one hardens. This is called moulting, and for those few hours the animal is soft, pale, and almost defenceless. Every insect, prawn, and spider you have ever seen has done this several times.

This is also why there is no insect the size of a dog. A bigger animal needs a thicker shell to hold itself up, a thicker shell is heavier, and a heavier one needs more muscle to move, which needs a thicker shell again. The design runs out of room. An animal with a skeleton on the inside, like a cow, has no such problem, because bone grows a little at a time and never has to be thrown away. So the arthropods won the whole world by number, and lost the competition for size.

A compound eye is not one eye with one lens, like yours. It is hundreds or thousands of tiny lenses packed together, each one pointing in a slightly different direction and each sending back a single dot of brightness. The picture that results is coarse, so a fly cannot read your face or see fine detail at all. What it can do is notice a change in any of those dots, instantly, from almost any direction, without turning its head.

That is exactly what a moving hand is. You are not being seen so much as detected, and from behind just as easily as from the front. This is the whole reason a fly is so difficult to catch, and it is a good example of a design that is poor at one thing and outstanding at another. Your eyes are the opposite way round: excellent detail in a narrow band in front of you, and nothing at all behind.

These animals have a soft body with muscular legs that help them move. They are found both in water and on land. Their head has soft, bendable feelers called tentacles. Most of them have a hard covering called a shell that protects their soft body, though some, like the octopus, do not have an outer shell. Male and female animals in this group are usually separate. The group of soft-bodied animals is called Mollusca. Snail, clam, and octopus are examples of this group.

A shell is a house you never leave, and like any house it is paid for. A snail is extremely well protected and extremely slow, because it is carrying its own armour everywhere it goes. It cannot run from anything, and it does not need to. An octopus belongs to the same group and made the opposite choice. With no shell it is soft and vulnerable, and in exchange it is fast, can squeeze through a gap the size of its own eye, and is one of the cleverest animals in the sea. Same group, two answers, and neither is wrong.

These animals live only in the sea. Their outer body is covered by a spiny skin, and unlike most other animals, they do not have a head. They swim and move using small, tube-shaped feet called tube feet, which work almost like tiny suction cups. Many of these animals have a strong power of regeneration, if an arm breaks off, it can slowly regrow. The group of spiny-skinned animals is called Echinodermata. Starfish and sea urchin are examples of this group.

Having no head is the strangest thing in this chapter, and it is worth asking why. Think about what a head actually is. It is the front end, the part that arrives somewhere first, so that is where an animal keeps its eyes, its feelers, and its mouth. A head exists because an animal moves forwards. A starfish does not have a front. It creeps slowly in whatever direction it chooses, so it has no use for a front end, and instead it senses in all directions equally, with arms arranged evenly around a centre. No head is not something missing. It is what a body looks like when there is no such thing as forwards.