Evolutionary trends
Conventional classification divides vertebrates into
two main groups—Gnathostomata, or vertebrates with jaws,
and Agnatha, or those without jaws (the lampreys and
hagfishes). This is a fundamental division, for agnathans
also lack paired fins and scales. Agnathans are regarded
as the most primitive group of vertebrates, not least
because they appear first in the fossil record, before
jawed fishes. Their circulatory systems differ in various
ways from those of jawed vertebrates.
