ULTRASTRUCTURE OF ENTERONEPHRIDIA AND GENERAL DESCRIPTION OF THE ALIMENTARY CANAL IN TROCHONERILLA-MOBILIS AND NERILLIDIUM-TROGLOCHAETOIDES (POLYCHAETA, NERILLIDAE)

Abstract

The alimentary canals of Trochonerilla mobilis and Nerillidium troglochaetoides consist of a ventral pharyngeal organ, oesophagus, stomach, intestine, and rectum. Prominent salivary glands lying lateral to the oesophagus discharge their secretions into the buccal cavity. Ciliated canals, the enteronephridia, embedded in the intestinal epithelium, open into the stomach near its border to the intestine. The ventral pharynx comprises a muscle bulb connected to a tongue-like organ by an investing muscle. The whole alimentary canal is ciliated except for the intestine of T. mobilis. The stomach is built up of absorptive cells and posteriorly also of secretory cells, whereas the intestine consists of only one cell type which is considered to be mainly absorptive. A typical microvillar brush border is present only in the intestine of T. mobilis; elsewhere the density of microvilli is low or the cells have irregular apical processes. In N. troglochaetoides the intestine has a ventral ciliary gutter laterally bordered by cells with highly specialized microvilli. The enteronephridia - 3 in N. troglochaetoides and 13 in T. mobilis - are unicellular tubes up to 130-mu-m long with a microvillar brush border and other cytological features typical for nephridial ducts. These structures are not known in any other polychaete taxon.