THE THREE-LAYERED ANIMALS 



173 



through fine tubules to the sperm duct 

 which opens to the outside through the 

 genital pore. The paired ovaries produce 

 eggs which pass through a small duct ( ovi- 

 duct) where they receive sperm from an- 

 other proglottid, or another worm, via the 

 vagina. Here they also obtain the food ma- 

 terial called yolk from the yolk gland, while 

 the shell gland secretes material for form- 

 ing the shell. The fertile eggs then are de- 

 posited into the uterus which eventually 

 becomes greatly distended as the eggs be- 

 gin to develop crowding all other structures 

 out of place. Such a gravid proglottid (full 

 of developing embryos ) breaks off and fol- 

 lows the cycle indicated above. 



Meat inspections in this country, together 

 with sanitation among cattle raisers, have 

 greatly reduced the incidence of this para- 

 site. In fact, it has become so difficult to 

 obtain this human parasite that biological 

 supply houses usually have standing orders 

 for them. Control of tapeworms is very 

 simple: merely cook the suspected meat. 

 There is a similar tapeworm in pork ( Taenia 

 solium) which is also becoming scarce. 



A few decades ago during the lumber- 

 jack days in this country, particularly in 



Minnesota and Michigan, the fish were 

 heavily infected with a tapeworm larva 

 that was transmitted to man through his 

 habit of eating raw fish. The worm, Diphijl- 

 lobothritwi latum, was apparently intro- 

 duced by immigrants from the Baltic re- 

 gion of Europe where the infection was 

 known for centuries. The life history in- 

 volves two intermediate hosts, a small crus- 

 tacean and a fish. The crustacean receives 

 the infection by eating tapeworm eggs, the 

 fish (pickerel or pike) eats the crustacean, 

 and finally man gets the worm by eating 

 the uncooked fish. This is a giant among 

 tapeworms, having been known to reach 

 a length of 60 feet. 



In review, we have seen that with the 

 advent of the mesoderm and with it several 

 important organ systems, the flatworms far 

 outstripped the coelenterates in complex- 

 ity. They are, however, still small creatures 

 and relatively simple when compared to 

 a mammal, in other words, still further im- 

 portant changes must have taken place in 

 subsequent groups of animals. We shall see 

 what additions are made in the next group 

 of tubular worms. 



