THE THREE-LAYERED ANIMALS 



169 



F!g. 9-10. Life history of a typical human blood fluke. 



flukes, the egg does not hatch into the larval 

 stage known as the miracidium until it is 

 eaten by a certain species of snail of tlie 

 genus Bijthinia. Inside the snail the egg 

 hatches, releasing the miracidium which 

 makes its way into the tissues of the snail 

 and develops into a sporocyst. The next 

 stage, the redia, develops inside the sporo- 

 cyst. Both the sporocyst and the redia 

 stages make possible a tremendous increase 

 in numbers by asexual reproduction. The 

 redia finally develops cercariae within its 

 walls, which make their way out of the snail 

 into the surrounding water where they 

 swim about by means of their large vibratile 

 tails. The cercaria then becomes attached 

 to the next host, one of several different fish 

 and, after losing its tail, bores its way into 

 the flesh of the fish. It rounds itself into a 

 ball and produces a cyst wall; in response 

 to the parasite, the fish secretes another 

 wall around the invader. Here it lies until 

 the raw fish is eaten by man in whose gut 

 the cyst wall is digested away, releasing the 

 young worm which makes its way up the 

 bile duct and finally into the smaller tubes 

 of the liver where it grows to maturity. 



The control of the disease is obviously 

 very simple, destroy the snails or cook the 

 fish, either of which interrupts the cycle 

 and kills the parasite. 



Some of the most important flukes are 

 the blood-inhabitino; schistosomes such as 

 Schistosoma haematobium (Fig. 9-10). Un- 

 like Opisthorchis, these worms are dioe- 

 cious, that is, there are two separate sexes. 

 They are long slender worms, beautifully 

 adapted for living in the small blood ves- 

 sels. A strange relationship exists between 

 the males and females; the male holds the 

 extremely slender female in a groove on his 

 ventral side, from which she ventures forth 

 during the business of laying eggs. Her 

 slender thread-like body is ideally adapted 

 to fit in the tiny blood vessels of the intesti- 

 nal wall or over the bladder where she lays 

 her eggs. The eggs have a single sharp spine 

 by which passively they work their way 

 through the wall into the cavity of the in- 

 testine or bladder where they are voided 

 with the urine or feces. 



Aeain through the use of human excre- 

 ment for fertilizer, the eggs usually find 

 their way into water. They hatch into mira- 



