630-600 million years ago
Flatworms evolved basement membranes and cilia in their epidermis, gonads, follicular cells around ovaries, and additional nuclear receptors.
The outer covering of the body of cnidarians is composed of a tissue
called epithelia, as is the case in all bilateran animals. The epithelia
which covers the body, known as the epidermis, can be ciliated in primitive
animals and used for locomotion. This trait of a ciliated epidermis is
known in diverse invertebrates, including basal deuterostomes and chordates
(Benito, form Harrison 1997, p. 16). Turbellarian flatworms possess a
basement membrane which joins their epithelia to the deeper layer of connective
tissue (Rieger, from Harrison, 1991).
The most primitive flatworms are equivalent to coelenterates with regards
to their reproduction. Acoela possess no gonads since spermatogonia and
oogonia are dispersed throughout the body. Eggs lack yolk (Hickman, Beklemishev,
vol. 2). Higher flatworms do possess gonads and in nemertine worms, the
gonads are lined by epithelium (Hickman). In some flatworms the ovary
is compacted and divided into layers so that oocytes may be surrounded
by follicular cells (Benito, form Harrison 1997, p. 93; Burighel, from
Harrison, 1997, p. 282).
Several nuclear receptors (a gene family which includes estrogen and testosterone receptors) are known from insects (such as ecdysone, ftz regulatory factor 1, and the products of the genes knirps, embryonic gonald, tailless, knirps, and ultraspiracle) and worms (the C. elegans differentiation activating factor). They all form part of a gene family derived from an ancestral protein with ligand-binding and DNA-binding domains (Amero, 1992).