Poisons are substances which cause death or injury when taken in by a living thing. It may be taken in as drink or food, or absorbed through the skin. The damage is usually done by a chemical reaction. The effect of the poison varies with the amount which is absorbed (taken in or inhaled). Substances which are poisonous are called toxic, but any substance can be toxic if too much of it is consumed. If poisoning causes death, it is "lethal" poison.
Legally, and in hazardous chemical labeling, poisons are especially toxic substances. Less toxic substances are labeled "harmful", "irritant", or not labeled at all.
In medicine and zoology, toxins and venoms are different from poisons. Toxins are the result of a biological process. Venoms are substances which the organism uses to harm other species. Certain organisms use venoms for hunting, or as a defense. If an organism is "poisonous", such as many mushrooms, it is harmful to eat. If it is "venomous", like snakes or honeybees, it has a harmful bite or sting. For some very deadly bites humans have developed effective antivenoms.
Often it is only the quantity of a substance that makes the difference. Drinking alcoholic drinks may lead to aggressive behaviour, problems with speech, and different forms of amnesia. This effect is called intoxication. People who drink even more may go into shock. At the same time, alcohol can be used as a disinfectant.
Sometimes, poisons have an antidote. The antidote of a poison will slow or reverse its effects. The antidote may itself be a poison. As an example, atropine can be used as an antidote against certain nerve gases, like tabun or sarin, or against certain insecticides. It is also used as a medication. In high doses, atropine is a poison. Yet atropine is a "core" medicine in the World Health Organization's "essential drugs list".
There are other types of dangerous materials. These are:
Pollution is also sometimes poisonous, for example toxic waste.
In wars, some countries use poison gases against their enemies. This is called chemical warfare.
Poison gases such as chlorine gas and mustard gas were used in World War I. In World War II, the Nazis used a form of hydrogen cyanide gas to kill many people in their death camps and concentration camps.
Poison gas has also been used to intentionally kill humans as method of death penalty.
Being poisoned by gas sometimes happens accidentally. For example, a faulty furnace or heating system can cause carbon monoxide poisoning.
There are many types of poison gases. For example, corrosive poison gases cause serious burns to the skin, eyes, and lungs. Nerve agents are poisons that can kill by damaging the central nervous system. Blister agents cause severe blisters on the inside and the outside of the body. Choking agents make fluid build up in a person's lungs until they drown.
In 2010, poisoning resulted in about 180,000 deaths, down from 200,000 in 1990. There were approximately 727,500 emergency department visits in the United States involving poisonings—3.3% of all injury-related encounters.
Powder
Powder is very small, dry pieces or grains. When we press some dry solids very hard, or when we cut something up into lots of small pieces, we can sometimes make powder.
For example, people sometimes put "face powder" on their faces, when they want to look good. Sometimes sports players put powder on their hands, to make them dry. We put "curry powder" in our food, to make it taste stronger. We put "washing powder" in the washing machine when we want to have clean clothes. People use "gunpowder" to make bombs and fireworks.
"Powder" is also a verb - meaning to apply powder.
Larva
Many animals develop in separate stages. A larva (plural: "larvae") develops from the egg in those animals. It is a separate life stage from the adult reproductive stage. A larva does not look like the adult animal, and changes shape (known as metamorphosis) as it grows up. There may be several larval stages before the adult form. Tadpoles, maggots and caterpillars are larvae.
Marine organisms with a larval stage often release large numbers of eggs and sperms into the water column. After fertilisation, the eggs develop into tiny larvae. The larvae develop and grow for some time before metamorphosing into adults. Most marine invertebrates and many fish have a pelagic larval stage or pelagic eggs. These live in the plankton, and may be transported long distances.
Probably the most widely accepted theory explaining the evolution of larval stages is the need for "dispersal". Sessile organisms such as barnacles and tunicates, and sea-floor groups like mussels and crabs, need some way to move their young into new territory, since they cannot move long distances as adults. Many species have relatively long pelagic larval stages (how long a larva is in the water column). During this time, larvae feed and grow, and many species move through several stages of development. For example, most barnacles moult through six "nauplius" larva stages before moulting to a "cipris", when they look to settle. The larvae eat different food from the adults, and disperse.
The other consideration is the small size of the eggs. If animals lay many small eggs (and most do), then the young stages cannot live the life the adults lead. They must live a separate life until they have the size and capability to live as an adult. This is what the larvae do.
A long-standing theory is that in transformed larvae of sea-squirts (tunicates) lies the origin of vertebrates. Tunicates are sessile, but their larvae are mobile, and have some features found in early vertebrates. The process of paedomorphosis, where juvenile features are retained in the adult, is the proposed mechanism. Genome analysis does show that the tunicates are the closest living relatives of vertebrates.
Amphibian
Amphibians are members of the class Amphibia. The living ones are frogs (including toads), salamanders (including newts) and caecilians. They are four-legged vertebrates which are cold blooded.
Amphibians lay their eggs in water, usually in a foam nest. After hatching they are tadpoles, which live in the water and have gills. The tadpoles change into adults in a process called metamorphosis. When they are adult, they have lungs to breathe instead of gills, and legs. Adult amphibians also use their skin to take in oxygen, and some species of salamanders do not have lungs.
The earliest amphibians evolved in the Devonian from lobe-finned fish which had jointed leg-like fins with digits. They could crawl along the sea bottom. Some had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if necessary.
For tens of millions of years, during the Carboniferous and early Permian, amphibia were top predators on land, especially in the low-lying tropical river systems. In drier conditions, they were less effective, and the ancestors of mammals and reptiles (the Synapsids and Sauropsids) gradually took over the land. They laid cleidoic eggs, which had hard shells, and could be laid out of water. Most of the early large amphibians went extinct in the Triassic period; a few survived to the Lower Cretaceous.
The only living amphibiana today are the Lissamphibia. These include the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians). They are all rather small, compared with mammals or reptiles. The smallest frog and vertebrate in the world is the New Guinea frog ("Paedophryne amauensis"). The biggest amphibian is the Chinese giant salamander ("Andrias davidianus").