190 Dynamic Theory. 



them is introduced into another stomach a man's, for example, by 

 being eaten raw (cooking kills them) when the operation is repeated. 

 When they are too numerous they produce disease, often fatal. 

 "Leuckart counted 700,000 trichinae in a pound of the flesh of a man, 

 and Zeuker speaks of even five millions in a similar quantity of human 

 fiesh. "* Trichinae of one variety or another may inhabit any warm- 

 blooded animal. (In some, however, they do not bore into the muscles 

 and become encysted, but pass out with the feces, and by becoming 

 attached to some article of food are taken into another stomach and fin- 

 ish their development.) The mouse harbors one which gets into a cat 

 when the cat eats the mouse. In the stomach of the cat it deposits its 

 young, which pass back to a mouse by becoming attached to some of its 

 food. It bores into the muscles of the mouse and is there encysted 

 ready to be eaten by another cat. ' Another parasite passes its life 

 between a mouse and a meal worm. The meal worm is the larva of the 

 Tenebrio Motitor, a coleopterous insect. It eats the feces of the mouse 

 and the eggs of the nematode parasite, which may chance to be enclosed 

 therein. These eggs hatch in the meal worm and bore through the 

 intestine into the layers of fat that surround it, and remain there while 

 the meal worm goes through his metamorphoses into the insect, which is 

 finally gobbled up by a mouse, in whose stomach the parasite lays its 

 eggs, and the cycle is complete. (Van Beneden, 247.) 



FIG. 94. Young Nematode Worm, 

 A single intestine from one end to the other. 



o Mouth. 



e Widening which may be called an esophagus. 



p Pharynx. 



d Middle intestine. 



a Anus. 



The Cestodes, or flat ribbon worms, differ from the 

 Nematodes, round worms or thread worms, notably in 

 |-cl this : that while the individuals of the Nematodes are 



always separate and independent from each other, the 

 Cestode individuals remain attached to each other and to 

 the mother that gave them birth. What is commonly 

 called a tapeworm, consisting of a head and a great 

 number of segments linked one after the other, is, in 

 reality, a colony, the head being a complete animal and 

 giving birth to each segment, one after another. These 

 segments grow and in each one a great quantity of eggs 

 are formed. The segment at the extremity opposite the 

 FIG. 94. head is the oldest and comes to maturity first. When 

 mature it is detached and escapes from the stomach of the host 

 in the feces. It is then nothing but a bag of eggs. The sarcode, 

 * Van Beneden, Parasites, 245. 



