THE WORM-LIKE ANIMALS 



341 



mouth or digestive organs, but absorb through their skin the 

 digested food in the alimentary canal of the host in which 

 they live. 



A tape- worm preserved and mounted in a jar containing 

 alcohol or formalin will show that the body 

 resembles a long, narrow ribbon or tape, 

 but is unlike a ribbon in that it is divided 

 into segments. The ribbon narrows near 

 the anterior end. The head is a rounded 

 knob, with a circle of hooks and four 

 suckers. There are nerves and excretory 

 tubes in each segment. Each of the larger 

 segments towards the posterior end has a 

 complete hermaphroditic reproductive sys- 

 tem, consisting of ovaries, spermaries, and 

 their ducts leading to the surface. The 

 extreme posterior segments become greatly 

 distended with enormous quantities of fer- 

 tilized eggs, each inclosed in a hard shell ; 

 and one by one these " ripe " segments 

 drop off and are carried out of the intes- 

 tine with indigestible matters or feces. As 

 segments drop off the worm, new ones are 

 formed by new grooves in the segments 

 back of the head. Thus the oldest seg- 

 ments are the most posterior ones. 



At the time a " ripe " segment is dis- 

 charged from the intestine, the egg-shell 

 contains a small embryo with six hooks. 

 If these small embryos, in the case of the 

 human tape-worms, happen to fall on grass or other food of 

 pigs and cattle, the digestive fluids in the stomachs of these 

 animals can dissolve the hard shell and free the embryo. 

 Then it bores into some organ and becomes encysted. Then 

 it develops a bladder-like structure with a tape- worm head. 



FIG. 107. A plana- 

 rian, flat worm. 

 The root-like in- 

 testine is shown in 

 black, e, eye; p, 

 pharynx; ra, mou!h. 

 Delicate excretory 

 tubes lie on either 

 side of the body. 

 (From Hatschek.) 



