Morphological and histological observations on digestive tract of Sipunculus nudus

Morphological and histological observations of the digestive tract in Sipunculus nudus were conducted with light microscopy.The digestive tract consists of mouth,pharynx,esophagus,intestine,rectum and anus.The mouth is surrounded by tentacles to form a mouth tray,and the pharynx is short and has no clear borderline with the mouth.The esophagus can draw back and forth.The intestine is very long and reverses three times to form a spiral.Caecum is attached to rectum.The wall of digestive tract is composed of mucosa,submucosa and adventitia,having no continuous muscle layer,only with numbers of muscle fibers in the submucosa.The epithelium of the mucosa membrane in digestive tract is mainly composed of columnar cells with developed microvilli except for intestine.The form of plica,the submucosa and the quantity of muscle fibers are also different among various parts of the digestive tract.