River otters primarily eat fish. They are also known to eat whatever is easiest to find, like crustaceans, mollusks, insects, birds, oysters, shellfish, crabs, crayfish, frogs, rodents, turtles and aquatic invertebrates.
1667: Robert Hooke 's famous Micrographia is published, which outlines Hooke's various studies using the microscope. 1675: Enter Anton van Leeuwenhoek, who used a microscope with one lens to observe insects and other specimen. Leeuwenhoek was the first to observe bacteria. 18th century: As technology improved, microscopy became more popular among scientists. Part of this was due to the discovery that combining two types of glass reduced the chromatic effect.
The microscope led to the discovery of cells. Most cells are microscopic, too small to see without the aid of a microscope. A microscope is an instrument which makes an object appear bigger than it is. It took the invention of this relatively simple tool to lead to the discovery of cells. In the 1660s, Robert Hooke began using microscopes to look at all sorts of materials. Anton van Leeuwenhoek took up similar work in the 1670s. They were among the first people to describe cells.