Zoology glossary ~ O

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Obligate
"Required; also, exhibited by all members of a species without exception."

Old World
"The Eastern Hemisphere, including Africa, Eurasia, and Australia."

Ondontocetes
A taxonomic suborder of toothed whales; this suborder includes sperm whales, white whales, porpoises, dolphins, and river dolphins.

Orbital Ring
"A fleshy ring around the eye; contrastingly colored in some species."

Origin of the Universe
Origin of the Universe

Oscines
"One of two major subdivisions of the order Passeriformes. Oscines have a more complex syrinx and distinctive DNA patterns and middle ear bone shapes, and they generally must learn their most complex vocalizations."

Osteichthyes
The class of fishes characterized by a swim bladder, bony skeletal systems, and an operculum.

Ostracum
The middle, shell building layer, of a snail's shell. Consists of prism-shaped calcium carbonate crystals and organic (proteid) molecules.

Oval
"Egg shape that is rounded at the largest end and tapered at the oterh end. Often used to describe the typical egg shape."