V FASCIOLA, SCHISTOSOMA 167 



Schistosoma (Bilharzia) 



One of the most important parasites of man is a small distomid 

 named Schistosoma (or Bilharzia) which lives in the veins of the body 

 and produces serious disease, the symptoms differing according to the 

 particular region of the body affected. The most conspicuous peculiarity 

 of the genus Schistosoma as compared with Fasciola is that the flattened 

 body has its edges curved towards the ventral side so as to take the 

 form of a cylinder, the wall of which is interrupted ventrally by a longi- 

 tudinal slit bounded by the edges of the body. The two sexes are strik- 

 ingly different in appearance (sexual dimorphism) the female being longer 

 and of much smaller diameter. When sexually mature the female appar- 

 ently lives permanently in contact with the male, gripped firmly between 

 the incurved edges of the latter's body (Fig. 77, A). The adult 

 worms live in the venous blood-stream and are often particularly con- 

 spicuous in the branches of the hepatic portal vein which run in the 

 mesentery. 



Our knowledge of the life-history of Schistosoma has been greatly 

 extended by the researches of Leiper carried out during the period of 

 the Great War and prompted by the dangers involved in the presence 

 of large masses of European troops in Schistosoma-infected regions such 

 as Egypt. We are now able to trace the life-history in its main outlines 

 and this will here be done in the case of Schistosoma haematobium. 



The eggs >are deposited in the blood-stream, especially in the small 

 veins of the bladder. The egg (Fig. 77, B, i) is somewhat ellipsoidal in 

 form, about 80 /x in length, and the shell is prolonged at one end into 

 a sharp pointed spine. No further development takes place unless it 

 reaches the exterior, and this it does normally by being passed from the 

 small blood-vessels of the bladder-wall into the cavity of that organ — 

 the sharp spine perforating the vessel wall and the egg probably being 

 forced out by the muscular contraction of the bladder-wall during the 

 final stages of the bladder being emptied. The urine in the bladder 

 thus comes to contain eggs along with a certain amount of blood, the 

 latter affording a conspicuous symptom (haematuria) of the disease. 



Further development of the egg is dependent upon its reaching fresh 

 water. After a variable period there hatches out a ciliated miracidium 

 (Fig. 77, C), elongated in form, possessing at its front, end a solid vestige 

 of alimentary canal {a.c), and on each side of this a large gland-cell {g). 

 Near the centre of the body is a nerve-ganglion {n.g) and on each side 

 a tortuous slender nephridial tube with two flame-cells («). 



The miracidium may live for a few hours but is incapable of 



