Ultrastructural studies of the epidermis and gastrodermis of Syndesmis franciscana (Turbellaria: Rhabdocoela)

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Abstract
Ultrastructural studies of the umagillid Syndesmis franciscana have revealed that it shares morphological characteristics with free-living turbellarians. The epidermis consists of a single layer of ciliated, columnar cells resting on a basal lamina. Organellar stratification is observed within these cells. Nuclei, mitochondria, and Golgi bodies are situated in the basal regions. Ciliary rootlets, microfilaments, and microtubules are associated with a terminal web-like zone in the apical areas. Neither epidermal nor subepidermal gland cells were observed in somatic regions. Rhabdites and pigment granules are lacking.The gastrodermis consists of a single layer of nonciliated columnar cells resting on a basement layer. The intestinal lumen is temporarily open. When food passes into the intestine, opposing luminal surfaces meet, causing local occlusions.The pharyngeal cavity, enclosed within the barrel-shaped pharynx, is lined by esophageal gland cell processes. Ciliates, identified as the sole food element, were found to be species of Entorhipidium. They are ingested and pass intact through the pharynx to the anterior intestinal lumen. In the intestine they were found in all stages of degradation within luminal, extraluminal, and intracellular vacuoles. Lipid droplets were frequently found in extraluminal vacuoles and in the ciliates present in such vacuoles. A mechanism for uptake of digested material is suggested by the occurrence of micropinocytotic vesicles associated with the extraluminal vacuoles.

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