This "acute schistosomiasis" is not, however, as important as the chronic forms of the disease. For S. mansoni and S. japonicum, these are "intestinal" and "hepatic schistosomiasis", associated with formation of granulomas around trapped eggs lodged in the intestinal wall or in the liver, respectively. The hepatic form of the disease is the most important, granulomas here giving rise to fibrosis of the liver and hepatosplenomegaly in severe cases. Symptoms and signs depend on the number and location of eggs trapped in the tissues. Initially, the inflammatory reaction is readily reversible. In the latter stages of the disease, the pathology is associated with collagen deposition and fibrosis, resulting in organ damage that may be only partially reversible.Granuloma formation is initiated by antigens secreted by the miracidium through microscopic pores within the rigid egg shell, and the immune response to granuloma, rather than the direct action of egg antigens, causes the symptoms. The granulomas formed around the eggs impair blood flow in the liver and, as a consequence, induce portal hypertension. With time, collateral circulation is formed and the eggs disseminate into the lungs, where they cause more granulomas, pulmonary arteritis and, later, cor pulmonale. A contributory factor to portal hypertension is Symmers fibrosis, which develops around branches of the portal veins.
The darker color is due to the presence of a pigment (hemozoin) in its digestive tube. This pigment is derived from the digestion of blood. The ovary is elongated and slightly lobulated and is located on the anterior half of the body. A short oviduct conducts to the ootype, which continues with the uterine tube. In this tube it is possible to find 1 to 2 eggs (rarely 3 to 4) but only 1 egg is observed in the ootype at any one time. The genital pore opens ventrally. The posterior two-thirds of the body contain the viteline glands and their winding canal, which unites with the oviduct a little before it reaches the ootype.The digestive tube begins at the anterior extremity of the worm, at the bottom of the oral sucker. The digestive tube is composed of an esophagus, which divides in two branches (right and left) and that reunite in a single cecum. The intestines end blindly, meaning that there is no anus. Sex
S. mansoni and other schistosomes are the only flukes or flatworms that exhibit sex separation as they exist as male and female individuals as in dioecious animals. However, they are not truly dioecious since the adults live in permanent male-female pairs, a condition called in copula. Although they can be physically separated,  isolated females cannot grow into sexually-mature adults. In copula starts in the liver only after which they can move to their final habitation, the inferior mesenteric veins. Individual females cannot enter the mesenteric veins.