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).