Hypothetical types of biochemistry in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life". Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes ("Chrysiogenes arsenatis").
Cavalier-Smith's system of classification had been subdivided into twelve phyla: The protozoa were originally classified as members of the animal kingdom. Now they are classified as multiple separate groups. Zoology is the study of animals while botany is the study of plants. While zoologists divided the animal kingdom into phyla, botanists carved the plant kingdom into "divisions". By 1940, five divisions were recognized: Fungi and bacteria were included within the plant division thallophyta. Today, bacteria are no longer classified as plants and fungi are known to be more closely related to animals than to plants. By mid-nineteenth century, microscopic organisms were generally classified into