To Be or Not to Be a Flatworm: The Acoel Controversy
Localization of BrdU-containing cells in the annelid Dorvillea bermudensis (A–C) and the nemertean Cephalothrix sp. (D–E) after 30 min incubation of BrdU. (A) Wholemount of the posterior segments of D. bermudensis. (B) Semithin cross section through midbody and parapodia of animal shown in (A). Labeled cells are located in the epidermis, the mesodermal space, and the gastrodermis (indicated by asterisk). (C) Magnified view of labeled epidermal cells shown in (B). (D) Anterior end of Cephalothrix sp. Labeled cells in the epidermis are separated from muscular layers and the cutis by a light-brown basal matrix. Labeled cells are also present in the mesodermal space. Inset shows details of labeled epidermal cells. (E) More posterior part of the animal than (D) with labeled cells in the epidermis. Arrowheads denote the proboscis. Scale bar is 100 µm for (A), 20 µm for (B), 5 µm for (C), and 50 µm for (D–E).