Ultrastructure of oesophageal appendages ("peptonephridia") in enchytraeids (Annelida : Clitellata)

Although the oesophageal appendages in the four enchytraeids Enchytraeus cr ypticus, Fridericia striata, Buchholzia appendiculata, and Achaeta sp. are quite different from one another in shape and position, their histology and ultrastructure are basically the same. These are intestinal appendages, th e lumina of which distally end blind and proximally open into the oesophagu s. Almost all of the few cells in their single-layered epithelium have a mi crovillous, cilia-free border at the apex, facing towards the lumen, and ba sally comprise an extremely extensive labyrinth. The presence of the latter , composed of very thin cell processes, and of numerous mitochondria identi fies the organs as energy-producing and -consuming, transport-active struct ures. Their possible function as a food-moistening organ or osmoregulatory organ is discussed, and they are compared with other intestinal appendages in enchytraeids and other oligochaetes.