Anatomical observation of the digestive tract in Phascolosoma pacificum Keferstein

This paper presented the form, gross structure and microstructure of the digestive tract of Phascolosoma pacificum. The digestive tracts could be divided into mouth, pharynx, esophagus, intestine, rectum and anus. As the principal part in digestive tracts, intestine was exceedingly long, and could fall into the descending intestine and ascending intestine. Both of the descending intestine and ascending intestine coiled mutually, making the intestine helix in shape. The wall of digestive tract consisted of mucosa, submucosa, tunica muscularis and tunica adventitia. The main differences among all parts lied in mucosa. The mucosa on pharynx and the superior segment of ascending intestine was stratified ciliated columnar epithelium, while mucosa on esophagus and rectum was monolayer ciliated columnar epithelium. Except the superior segment of descending intestine, mucosa of intestine and all other segments was monolayer columnar epithelium. There were inverse u-shaped ciliated apophysis and v-shaped ciliated grooves on the inferior segment of descending intestine, while only v-shaped ciliated groove existed on the ascending intestine. Besides, a great deal of granulous cells occurred on the epithelium of the inferior segment of descending intestine and the superior segment of descending intestine. The form of plica, the submucosa and the thickness of tunica muscularis were also different in various parts of the digestive tract. This study showed that the digestive tract structure of P. pacificum was adapted to its feeding habits.