ULTRASTRUCTURAL STUDIES OF THE VISCERAL MUSCLES OF CHAETOGNATHS

The visceral musculature of Chaetognaths was studied with special atte ntion given to the digestive apparatus muscle. In the head the digesti ve apparatus muscle is relatively thick; individual muscles are diffic ult to distinguish at the anatomical level; in the anterior part of th e oesophagus discontinuous bundles and layers of cross-striated fibers are found. As a group however, the oesophageal musculature completely covers the oesophageal epithelium. The prominent muscle layers around the oesophagus probably help to force food into the intestine against the turgor pressure of the trunk cavity which tends to collapse the i ntestine. Around the intestine the musculature is largely circular and smooth. The intestinal epithelium is ciliated despite its muscular co vering. Muscle fibers are not individually innervated. They form myoep ithelial structures with various intercellular junction types. In the intestinal muscle the fibers show myoendothelial-like junctions. Sphin cters composed of myoepidermal cells surround the anus and the female gonopores. The somatic side of the general cavity is lined with a poly morphic squamous epithelium. Sometimes myoepithelial cells are found, with the occasional presence of extracellular matrix basal to the laye r of the squamous epithelium. The ontogenetic relations between the po lymorphic epithelium and the composite 'mesenteries' remain to be esta blished. We have now some idea about the architecture of the body of C haetognaths in relation to contractile structures.