Morphological Structure Characteristics Of Digestive Tract And Nephridium In The Sipunculid Phascolosoma Esculenta

Phascolosoma esculenta,which is one of the Sipunculoidea economic animals,is with verygood value of edible and medicine use.For the past few years, the breeding industrialization ofPhascolosoma esculenta had succeed in the coast of Zhejiang,Fujian and other places. With thepurpose of exploring the structural features of digestive and excretory organs and accumulatingelemental information for the further research of digestive biology and excretory mechanisms,Histology and cytomorphology of digestive tract and nephridium in this species were observed indetail by microanatomy and electron microtechnique.Main results and conclusion were as follows:1. Histological observations of digestive tract in Phascolosoma esculenta The digestivetract begin with mouth,via pharynx, esophagus, intestine, rectum and stop at anus,no caecus andspecial digestive gland were found. The long intestine coils are around a long, slender, centralspindle muscle to form a helix hanging in the body cavity. Mouth is located on the top of theintrovert,neither teeth nor tongue,and there are tentacles dorsally;There is no distinct demarcationbetween the mouth and thepharynx; The esophagus is embedded in or adhere to the retractormuscles. From outside to inside,the wall of digestive tract is composed of adventitia,muscularis,submucosa and mucosa.The epithelium of pharynx are pseudostratified ciliatedcolumnar epithelium with of a small amount of goblet cell, and there are ciliary epipharyngealgroove between the plicas. The mucous epithelium in the forepart of esophagus arepseudostratified ciliated columnar epithelium,but simple columnar epithelium in the posteriorsegment,and the mucosa are well-developed;From front to back,the number of intestinal villus isincreased and the plicas are from low to high and then heightened, the entericepithelium are allsimple columnar epithelium and with goblet cell.There are muscularis mucosa in the mucous andwell-developed microvilli and cilium on the surface of the epithelium cell in dissociatingintestine.In the superior segment of descending intestine, there are well-developed goblet cell andmucous cells in the mucous epithelium of anterior segment and huge amounts of microvilli andcilium on the dissociative side of epithelium cells in the posterior segment;In the inferior segmentof descending intestine,the gut cavity fills out largen,plicas in the anterior segment are maximumand highest;the mucosal folds are lower obviously and with fewer goblet cell and very flourishing microvillis on the dissociative side of epithelium cell in the posterior segment. In the inferiorsegment of ascending intestine, the gut cavity are even bigger, the mucosal folds are not obviousand the submucosa are thin. In the superior segment of ascending intestine,the circular muscle layerand adventitia thicken and villus plicas become flourishing,and goblet cell and mucous cellsincrease.The ciliary grooves are present in both descending intestine and ascending intestine.In theanterius segmentum of rectum,the mucosa plicas increase and with goblet cell and mucouscells,there are ciliums on the dissociative side of epithelium cell;but the mucous epithelium in theposterior segment of rectum are composed of short single ciliated columnar epithelium with a fewof goblet cells. There are granule cells all over the intestine and most of which in the dissociateintestine and the superior segment of ascending intestine. Analyses show that:The features of longand coiled highly of the intestinal tract can lengthen the time of storing, digesting and absorbing thefood.The goblet cells and granule cells in the digestive tract can produce mucus which can play apart in conglutinating and lubricating the food grain and residue,as well as the swing of cilium inthe mucosa and the existence of ciliary groove are conducive to the food passing through thedigestive tract.The foods are mainly digested in the middle and inferior segment of the descendingintestine and absorbed in the middle and ferior segment of ascending intestine.All the resultsindicate that digestive tract of Sipunculan are accommodative to its feeding habits.2. Histological and cytomorphological observations of nephridium in Phascolosomaesculenta The nephridium is covered with large number of flask-shaped infoldings on theexternal surface and can be divided into a funnel,a excretory sac and a excretory tube.There arevery more flask-shaped infoldings on the surface of excretory tube than on the surface of excretorysac. From inside to outside,the wall of the nephridium consists of simple columnar epithelium orsimple cuboidal epithelium,the nephridial musculature, connective tissue and adventitia. Thecolumnar epithelium arrange into bunches and insert microvilli and cilia on the dissociative sides;The basal surface plasmalemma of which invaginate and form the basal labyrinth;The cubical andflattened cells line the lumens of the flask-shaped infoldings and bear microvilli and cilia on thedissociative sides;There are well-developed organelles and numerous granules with different electrondensities,size and shape in the cytoplasm of the columnar and cuboidal epithelium. Within the matrixthere is a muscular grid consisting of longitudinal and circular cords; The connective tissue,which iscomposed mainly of interlacing collagenous fiber,forms a reticulate structure and withgranulocytes. The adventitia consists of the system of podocytes and multiciliary cells. Accordingto the morphological structure features of the nephridium,two zones of filtration and excretionthrough the wall of excretory tube have been found:①through the surface of flask-shapedinfoldings(via ssuccessively diaphragms of podocytes,connective tissue consists of reticular fibers,microvillary canals between cubical or flattened epithelium cell of flask-shaped infoldings,then into the lumen of flask-shaped protrusion), and②through the areas between the flask-shapedinfoldings and the basal labyrinth(via the diaphragms between processuses of podocytes,thickconnective tissue consist of reticular fibers, and the basal labyrinth of the columnar cells, then enterinto the columnar cells).These datas on the anatomy and histology allow us to conclude that, exceptfor excretory function the nephridium plays a part in accumulation and excretion of gametes and workslike a gonoduct.