THE GROUPS OF ANIMALS 267 



worms which are introduced in insufficiently cooked pork. The pigs get 

 it by eating meat refuse or infested rats. The larvae get into the lym- 

 phatic vessels or bore out through the intestinal wall and enter the mus- 

 cles, where they become encysted (Fig. 224A). Government inspection 

 of meats is carried out in a few countries, but in some of those with the 

 most rigid inspection the incidence of trichinosis is high. The reason is 

 the habit of eating rare pork in those countries. Thorough cooking is 

 the safest preventive; once the larvae are on their way to the muscles there 

 is no cure. Members of another family of roundworms may cause ele- 

 phantiasis by clogging the lymph passages. 



The hookworm (Fig. 2245) of the southern states is also a member of 

 this phylum. The larvae develop in moist soil. From there they enter 

 the body through the skin of the feet, get into the blood, and thus reach 

 the lungs and intestines. By feeding upon the blood and causing bleed- 

 ing through an inhibition of clotting they produce an anaemic condition. 

 Injury to the lungs predisposes the victim also to tuberculosis. The 

 shiftlessness of the ''poor whites" in the South is attributed in part to 

 hookworm disease. An important feature of preventive measures is 

 proper disposal of human feces, so as to prevent pollution of the soil, thus 

 stopping further infection. Curative treatments are also available for 

 those already diseased. 



Phylum 6. Echinodermata. — Members of this phylum are radially 

 symmetrical in the main, though usually some small feature is eccentri- 

 cally placed so as to introduce slight bilaterality. Usually there are five 

 rays, but the number may be very much greater. The skeleton consists 

 of limy plates, either firmly joined into a globular shell or more loosely 

 aggregated in the body wall so as to be readily movable on one another. 

 There is a distinct coelom. Many echinoderms possess a peculiar method 

 of locomotion by means of tube feet. These are hollow muscular tubes, 

 ending in suckers and filled with water by which they are operated. The 

 tube feet may be thrust out long distances by pressure on the contained 

 water, attached to fixed objects by the suckers, then contracted, pulling 

 the whole animal slowly along. Locomotion is more rapid in the brittle 

 stars, since the slender arms of these animals can be bent rapidly and pro- 

 vide a sort of walking or running movement. Some of the feather stars 

 are sessile^ being attached by a jointed stalk to the bottom. All members 

 of this phylum are marine. 



Starfishes (Fig. 225 A) have arms usually well marked off from the 

 body disk. The brittle stars (B) have this distinction of arms from the 

 body disk especially clearly marked. The name brittle star comes from 

 the animals' practice of breaking off injured arms, which thereupon 

 regenerate. 



Sand dollars (D) have a nearly smooth margin, without division into 



