Parasitism and Social Reciprocity. 191 



or contractile skin of this bag, has no other function than to hold 

 these eggs, and when it is eaten, or when it breaks up, as it commonly 

 does upon exposure to the air, the eggs escape and are scattered upon 

 the grass or in water, or are liberated in the stomach of a host. In any 

 event they must get into a stomach again in order to continue their ex- 

 istence. And they may reach the stomachs of cattle, rabbits, sheep, 

 &c. , by being swallowed with the grass. A pig may get them from 

 various sources, and man may get them from water into which they 

 have been washed or discharged. In the stomach the egg, which is al- 

 ways enclosed in a strong and nearly impenetrable shell, is hatched, or 

 at least liberated from the shell, the latter being dissolved or digested 

 by the gastric juice, or otherwise, and the worm thus liberated starts to 

 bore through the walls of the intestines and the tissues till it reaches a 

 proper shelter in the cavity of a muscle or other organ, where it remains 

 till the organ is eaten and they are introduced a second time into a 

 stomach. Then, being liberated from the tissue that they are enclosed 

 in, they attach themselves to the coat of the stomach and begin to grow 

 one segment after another of the sarcode bags described above, in which 

 the eggs are formed and encysted in their strong shell. It is necessary 

 to carefully distinguish between the eggs, as they are when thrown off 

 in the feces of an animal with a tapeworm in the stomach, and the 

 cysticercus, which is the name given to the animal after it has bored 

 from the stomach into a muscle and become encysted there. 



The egg always produces the cysticercus before it can grow into a 

 tapeworm. The formation of the cysticerci is often attended by dis- 

 ease when they are numerous. In the pig the disease is called the 

 measles. In the sheep the disease called the "gid" is caused by the 

 embryos of teniae encysted in the brain. In man the presence of these 

 cysticerci in the brain has caused mental derangement. But in all 

 cases where the cysticerci get into the stomach after having been devel- 

 oped and embedded in muscle or other tissue, the tapeworm is the re- 

 sult. There are many different varieties of these tenise, depending on 

 the difference in the animals they frequent. The one that forms its 

 segments and eggs in the stomach of man, and its cysticerci in the mus- 

 cles of the pig is the Tcenia Solium. The Tcenia Serrata rotates through 

 the stomach of dog or wolf and the muscles of rabbit. The Tcenia Me- 

 dio-canellata uses the stomach of man and the muscles, lungs, &c. , of 

 beef cattle, and has been confounded with the T. Solium, which it re- 

 sembles. The one which lodges in the brain of the sheep and develops 

 into a tsenia in the stomach of the dog or wolf is called the Ccenurus, a 

 name given before its nature was fully known. There are many others 

 of which Van Beneden has given ample account. 



From what was said in last chapter it appears that the relationship of 



