542-488 million years ago


The ancestor of modern urochordates (such as in the tunicate pictured above) and vertebrates evolved a number of additional modifications such as the use of the pharyngeal slits as gills, a corpus luteum in the female reproductive system, and various junctions between cells in the epidermis.

The blood vessels to pharyngeal slits are more extensive in urochordates than hemichordates (Harris). In hemichordates, they are primarily used in feeding. In urochordates, these structures perform gas exchange and function as part of the internal gills around the pharynx (unlike the external gills observed in other groups of invertebrates; Romer, p. 348).

In tunicates, some of the follicular cells remain as a "corpus luteum" or follicular residuum after ovulation. (Burighel, from Harrison, 1997, p. 288).

In tunicates tight junctions, gap junctions, and desmosomes exist in the epidermis (Burighel, from Harrison, 1997, p. 232).