THE FLATWORMS— PLATYHELMINTHES 149 



There is a reason for this emphasis on reproduction ; the life cycle is 

 very precarious and the chances of any one egg completing it and getting 

 into another primary host are quite small. They make up for this by 

 giving over their bodies in a large measure to the production of eggs, 

 and the necessary sperms. Thus, enough of them will complete the cycle 

 to keep the species going. A beef tapeworm, for instance, may live in 

 your body for ten years and produce more than 250 million eggs each 

 year during this time. If we assume that the number of beef tapeworms 

 is not increasing or decreasing, that would mean that any one egg has 

 one chance in 2,500 million to carry out its destiny to proper conclusion. 

 Nature achieves her ends by devious and remarkable methods. 



A good example of this class is the pork tapeworm, Taenia solium, 

 which is very common in sections of Europe and other parts of the world 

 and at one time was very prevalent in the United States, but our efficient 

 system of meat inspection has made it comparatively rare today. 



As an adult this tapeworm is found in the small intestine of man and 

 normally reaches a length of from six to ten feet with a width of about 

 two fifths of an inch at the widest portion which is at the posterior end. 

 This may seem like a great length for man to accommodate, but the human 

 small intestine is about 24 feet long so there is plenty of room for several 

 of these tapeworms without crowding. They do some harm by absorbing 

 food intended for human nourishment, but the main damage seems to be 

 due to an absorption of their waste products which may cause anemia, 

 nervous disorders, and other troubles. Also, on some occasions they may 

 congregate in one portion of the intestine to such an extent as to cause 

 obstruction which interferes with the normal passage of materials 

 through the intestine. 



Although they continue to produce new proglottids from the neck of 

 the scolex throughout their life, they do not increase in length after they 

 have reached maturity, because proglottids break off at the posterior end 

 about as fast as they are produced at the anterior end. When a pro- 

 glottid is first produced at the neck it is small ; as it is pushed away from 

 the neck by new proglottids being formed it grows in size and develops 

 its sexual organs. The male organs seem to mature first and release the 

 sperms into the intestine so that they can swim down and fertilize mature 

 eggs in other segments. Then the eggs mature and are fertilized by in- 

 coming sperms and shortly begin their embryonic development within the 

 uterus. As more eggs are produced the uterus swells until it fills the 

 proglottid and it is bulging with young embryos. By this time the pro- 

 glottid has reached the posterior end of the tapeworm and is said to be 

 ripe. Then, together with several other segments, it breaks loose and 

 is carried from the body in the feces. 



