The octopus has been studied in the laboratory and has proved to be highly intelligent. Experiments have shown that the octopus can recognize shapes and colours, and can remember experiences. This means that it can be trained to some extent. The octopus also demonstrates its intelligence by escaping from captivity by climbing out of almost any tank in which it is kept.
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Ribbon worms are marine worms that can grow as long as 30 m. they capture their prey by shooting out a spiked organ called a proboscis. It impales the prey so the worm can haul it in and eat it.
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Cuttlefish and squid have a visual language in which they communicate by changing their body colour and patterns. This change happens extremely quickly. Waves of colour and complicated patterns pass over the animal’s body when it is frightened or hungry, or trying to escape from danger.
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Most long, thin-bodied animals are called ‘worms’, but they are not all closely related. Roundworms are the most common type of worm. At least 15,000 different types of roundworm have been described, and there could be as many as 500,000 species altogether. Many roundworms live in the soil, and there may be millions in one cubic metre of soil. They feed on dead and decaying material. Other worms are predators or parasites, which live inside the bodies of other animals.
Worms are found in almost any damp environment. Some marine worms live in tubes made of sand, sieving the water with flower-like tentacles. Leeches are aquatic worms that may such blood. Most leeches are aquatic, but some live in tropical rainforests and wait on the surface of the vegetation until they can attach themselves to a passer-by. They drop off after feeding.
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Invertebrates, or animals without backbones, make up 97 percent of all animals. This group of animals includes all the arthropods such as spiders, insects and crustaceans. The remaining invertebrates are mainly soft-bodied animals, although many of them have shells. They include animals such as sponges, corals, shellfish, worms, sea urchins and starfish and many less familiar animals.
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The biggest crab is the giant spider crab found in the Pacific Ocean, near Japan. The crab lives at a depth of up to 300 m. when it claws are outstretched, they can span as much as 3.6 m. the giant spider crab’s body is actually quite small and is only 37 cm across, but the whole animal may weigh as much as 18 kg.
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Crustaceans are aquatic arthropods such as crabs, lobsters and shrimps. They have a very tough, jointed external skeleton and jointed walking legs. Their body is divided into a region that contains most of the internal organs, covered by a shell called the carapace, and a muscular tail section that is usually folded under the body. Many crustaceans have powerful pincers that they use to capture and break up their prey, and to signal to others of their species. Crustaceans live in the sea and in freshwater habitats.
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The feet of a fly are adapted so that they can stick to almost any surface by means of a combination of tiny hooks and sticky pads. A fly’s feet can even grip on shiny surfaces such as glass. When a fly lands on a ceiling, it flies up towards it, and then somersaults into a half-loop so that it finishes in an upside-down position, ready to grasp the ceiling.
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Insects have had 340 million years of experience in flying, which they now do very efficiently. Insects have two pairs of wings, which are flapped using a different kind of mechanism from birds. There are several ways in which the wings can beat, but usually they are raised by the contraction of a muscle. The wing then snaps down powerfully due to the spring-like properties of the insect’s external skeleton. This mechanism allows the wings to beat very fast – 250 times per second for bees, and as many as 1,000 times per second for some midges (which accounts for the high-pitched whine these insects make).
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Insects such as grasshoppers and locusts emerge from eggs as miniature adults. Other insects go through a complicated development during which they completely change their shape and structure. Houseflies, bees, wasps, butterflies and moths go through various stages of development. First they produce a larva, such as a grub or caterpillar, which lives an independent existence for some time. Next the larva goes through a process which is called metamorphosis. This is when the animal rests in a protective cover called a pupa. Inside the pupa, the insect’s internal structure is gradually rearranged into the adult form. Finally, the pupa splits open and the adult insect breaks out – it looks totally different to its larval stage. At first its body is very soft and vulnerable to predators, but the hard exterior skeleton soon forms. The adult is now ready to breed.
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Together with scorpions, ticks and mites, spiders are part of a group called arachnids. They are not insects. The body of an arachnid is divided into two parts, while the body of an insect is in three sections. Arachnids have four pairs of walking legs, while insects have three pairs. Spiders are the most common members of this group of arthropods, and they are all carnivores. They capture their prey in webs or by running after them, paralyzing them by injecting poison from their jaws. Spiders cannot swallow solid food; instead they inject digestive juices into their prey, which liquefy it. They are then able to suck in the digested food through their jaws.
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Spiders spin their webs from silk. It is pumped out from tiny nozzles at the back of the abdomen, which are called spinnerets. As the silk is stretched by the spider into a thread, it hardens. For its thickness, the thread becomes proportionately stronger than steel. Some of the threads are sticky, while others simply support the web. The spider is able to feel the vibrations of the web when an insect flies into it, and it runs quickly across the web to capture and subdue the prey. Usually, the insect is wrapped in silk before being eaten by the spider.
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The rigid shell of arthropods cannot stretch to allow the animal to grow, so the shell must be discarded. When the shell splits open, the animal emerges and withdraws its legs and antennae. The newly emerged animal is soft and can now increase in size. A new hard shell forms within a few days. The animal is vulnerable to predators during this period, and must hideaway while the new shell develops.
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Arthropods are animals with a hard external skeleton like a suit of armour. The skeleton is jointed to allow movement. Arthropods have evolved in a different way to vertebrates and even their blood is chemically different and so it is not red. They do not have a brain and spinal cord like vertebrates. Instead, they have a nerve cord running along the underside of their body, and small thickenings of this nerve cord instead of a brain. Arthropods have efficient eyes, but these work in a different way from those of vertebrates. Arthropods such as spiders may have many eyes.
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Sharks kill several people every year, probably because they mistake swimmers for seals, which are often their normal prey.
The small piranha can also be extremely dangerous. Piranhas live in rivers in South America. They are stocky disc-shaped fish, about 30 cm in length, with a blunt head. Piranhas feed by biting out a crescent-shaped piece from their prey with razor-sharp interlocking teeth. They can give a nasty bite, but are not really dangerous unless they attack in large numbers. Piranhas are supposed to be able to strip all the flesh from a pig or cow in a few minutes, but they are probably not dangerous to humans unless they are attracted by blood.
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