What is the origin of 'sleep tight'?
Read moreOne of the mysteries of the English language finally explained.
Zoology. A member of a group of animals from which vertebrates evolved; an animal considered to represent, or to be related to, an evolutionary ancestor of the vertebrates; a very primitive vertebrate.
Zoology. Of or relating to a protovertebrate or protovertebrates.
Late 19th century; earliest use found in Theodore Gill (1837–1914). From proto- + vertebrate.