AP BIOLOGY

Detailed explanation-1: -Using single-lensed microscopes of his own design and make, van Leeuwenhoek was the first to observe and to experiment with microbes, which he originally referred to as dierkens, diertgens or diertjes (Dutch for “small animals” [translated into English as animalcules, from Latin animalculum = “tiny animal"]).

Detailed explanation-2: -Van Leeuwenhoek discovered “protozoa”-the single-celled organisms and he called them “animalcules". He also improved the microscope and laid foundation for microbiology. He is often cited as the first microbiologist to study muscle fibers, bacteria, spermatozoa and blood flow in capillaries.

Detailed explanation-3: -Leeuwenhoek looked at animal and plant tissues, at mineral crystals and at fossils. He was the first to see microscopic foraminifera, which he described as “little cockles. . . no bigger than a coarse sand-grain.” He discovered blood cells, and was the first to see living sperm cells of animals.

Detailed explanation-4: -Answer and Explanation: “Animalcule” is an old term for microorganisms.