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shark

(Encyclopedia)shark, member of a group of almost exclusively marine and predaceous fishes. There are about 250 species of sharks, ranging from the 2-ft (60-cm) pygmy shark to 50-ft (15-m) giants. They are found in ...

monotreme

(Encyclopedia)monotreme mŏnˈətrēmˌ [key], name for members of the primitive mammalian order Monotremata, found in Australia, Tasmania, and New Guinea. The only members of this order are the platypus, or duckbi...

ovary

(Encyclopedia)ovary, ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone, which control t...

brine shrimp

(Encyclopedia)brine shrimp, common name for a primitive crustacean that seldom reaches more than 1⁄2 in. (1.3 cm) in length and is commonly used for fish food in aquariums. Brine shrimp, which are not closely rel...

sea urchin

(Encyclopedia)sea urchin, spherical-shaped echinoderm with movable spines covering the body. The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles...

birth control

(Encyclopedia)birth control, practice of contraception for the purpose of limiting reproduction. Although contraceptive techniques had been known in ancient Egypt, Greece, and Rome, the modern movement for birth ...

uracil

(Encyclopedia)uracil yo͝orˈəsĭl [key], organic base of the pyrimidine family. It was isolated from herring sperm and also produced in a laboratory in 1900–1901. When combined with the sugar ribose in a glycos...

diatom

(Encyclopedia)diatom dīˈətŏmˌ, –tōmˌ [key], unicellular organism of the kingdom Protista, characterized by a silica shell of often intricate and beautiful sculpturing. Most diatoms exist singly, although s...

lamp

(Encyclopedia)lamp, originally a vessel for holding oil or some combustible substance that could be burned through a wick for illumination; the term has been extended to other lighting devices. Stones, shells, and ...

Down syndrome

(Encyclopedia)Down syndrome, congenital disorder characterized by mild to severe mental retardation, slow physical development, and characteristic physical features. Down syndrome affects about 1 in every 730 live ...