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Defending yourself is absolutely crucial when you are an animal preyed upon by a predator or a plant that is attacked by herbivores, and it is of no minor importance to be able to resist infection by pathogens or parasites. Due to the ubiquitous nature of these problems but limited possibilities for solving them, it is no wonder that very different groups of organisms have arrived at very similar solutions.
When it comes to defence against being eaten, one mechanism employed is to make yourself invisible to predators. Such camouflage is often visual, and the octopus for example is a master of melting into its environment. Visual camouflage can be found in numerous other groups of animals (and even in plants!) and sometimes even involves decorating yourself with objects from your surroundings. Decoration has evolved independently in, for example, spider crabs and a species of weevil that both host a garden of algae and other organisms on their back. Astonishingly, camouflage can also be olfactory - several species of mammal (e.g. ground squirrels, Siberian chipmunks, hedgehogs and tenrecs) anoint themselves with the scent of poisonous animals, such as snakes or toads.
To avoid ending up as a meal many organisms are toxic, and this approach is rampantly convergent, too. The sequestration of chemicals such as alkaloids for defensive purposes has evolved numerous times, such as in frogs, many insects, myriapods, some birds (e.g. the New Guinean pitohuis), tunicates and, of course, plants. Toxicity is often correlated with bright warning colours (referred to as aposematic colouration) that signal to potential predators. In frogs alone, this correlation between toxicity and colouration has evolved several times. Harmless organisms regularly exploit the aposematic colours of toxic or unpalatable species by mimicking them. This type of mimicry is known as Batesian mimicry, whereas in Muellerian mimicry aposematic organisms mimic each other.
Alternative means of defence are protective structures, such as spines. Defensive spines can be found in numerous animals (e.g. sea urchins, insects, some fish, lizards, hedgehogs, porcupines and other spiny mammals) and, of course, in many groups of plants. In some eusocial insects (eusociality representing a key example of convergence in itself), defence of the colony is undertaken by a specialised soldier caste, characterised by features such as large body size or massive mandibles.
Defensive secretions are widespread, including ink in cephalopods and sea-hares (Aplysia), foam in several groups of insects (most famously the spittlebugs but also in some grasshoppers and moths) and latex that is produced by many plants from different groups to gum-up attackers. Latex additionally illustrates that the evolution of counter-defences can be convergent, too - a number of insects have independently arrived at methods of sabotaging the plants' lactiferous system.
In defence against pathogens and parasites, elements of an immune system have evolved not only once but several times. The non-specific innate immune system can be found in many animals as well as in plants, but even the highly specialised adaptive immune system has not only been invented by the jawed fish. It has recently been discovered that jawless fish have a similar system that relies on different mechanisms and components, and also the insect immune system shows at least some parallels.