=There are four classes in the branch Vermes=: 1) the _earthworms_, including sandworms and leeches; 2) the _roundworms_, including trichina, hairworms, and vinegar eels; 3) _flatworms_, including tapeworm and liver fluke; 4) _rotifers_, which are mere specks in size. [Illustration: FIG. 80.--HEAD OF SANDWORM (enlarged).] The =tapeworm= is a flatworm which has lost most of its organs on account of its parasitic life. Its egg is picked up by an herbivorous animal when grazing. The embryo undergoes only partial development in the body of the herbivorous animal, _e.g._ an ox. The next stage will not develop until the beef is eaten by a carnivorous animal, to whose food canal it attaches itself and soon develops a long chain of segments called a “tape.” Each segment absorbs fluid food through its body wall. As the segments at the older end mature, each becomes full of germs, and the segments become detached and pass out of the canal, to be dropped and perhaps picked up by an herbivorous animal and repeat the life cycle. The =trichina= is more dangerous to human life than the tapeworm. It gets into the food canal in uncooked pork (bologna sausage, for example), multiplies there, migrates into the muscles, causing great pain, and encysts there, remaining until the death of the host. It is believed to get into the bodies of hogs again when they eat rats, which in turn have obtained the cysts from carcasses. =Summary of the Biological Process.=--An earthworm is _a living machine which does work_ (digging and crawling; seizing, swallowing, and digesting food; pumping blood; growing and reproducing). To do the work it must have a continual _supply of energy_. The energy for its work is set free by the protoplasm (in its microscopic cells) undergoing a destructive chemical change (_oxidation_). The waste products from the breaking down of the protoplasm must be continually removed (_excretion_). The broken-down protoplasm must be continually replaced if life is to continue (the income must exceed the outgo if the animal is still growing). The microscopic cells construct more protoplasm out of food and oxygen (_assimilation_) supplied them by the processes of nutrition (eating, digesting, breathing, circulating). This protoplasm in turn oxidizes and releases more energy to do work, and thus the cycle of life proceeds. CHAPTER VII CRUSTACEANS CRAWFISH SUGGESTIONS.--In regions where crawfish are not found, a live crab may be used. Locomotion and behavior may be studied by providing a tub of water, or better, a large glass jar such as a broad candy jar. For suggestions on study of internal structure, see p. 58. =Habitat.=--Do you often see crawfish, or crayfish, moving about, even in water where they are known to be abundant? What does your answer suggest as to the time when they are probably most active?