ROUNDW ORMS— THE NEMATHELMINTHES 155 



homemade pickles and other similar foods prepared with apple-cider 

 vinegar it might be best if you do not examine the scum that forms on 

 the top of the liquid, for you are very likely to find the same wiggling 

 little worms. These are called vinegar eels and do man no harm. Since 

 you, no doubt, have eaten many of them in the past, there is no reason 

 that you should deny yourself these foods in the future. There is one 

 species found in Germany that lives a life that some people would envy; 

 they inhabit the mats upon which the mugs of beer are placed and live 

 entirely on the beer that slops over edges of the mugs. 



In addition to these very abundant free-living forms, the roundworms 

 are, by far, the most abundant of the metazoan parasites. Some are 

 plant parasites, crawling around among the cells, sucking the cell sap, 

 and causing the plants to wither and otherwise interfering with their 

 normal growth. Vertebrate animals usually carry from one to half a 

 dozen species in their bodies at all times, with the exception of civilized 

 man, who, by sanitation and medical vigilance, manages to avoid such a 

 universal infection ; yet there are comparatively few persons who have 

 not harbored at least one of these parasites during their lifetimes, perhaps 

 unknowingly. 



The Large Roundworm of Man — Ascaris 



As a type animal for this phylum, we will study Ascaris which is a 

 large roundworm that lives in the intestine of man. It is probably the 

 oldest known parasite of man because it is so large that it could hardly 

 be overlooked when passed from the body, even by prehistoric people. 

 It may be about a foot long and normally stays in the small intestine, but 

 it seems to be a very inquisitive creature and sometimes crawls into the 

 liver, pancreas, appendix, or occasionally all the way up the esophagus 

 and out of a person's nose to his great horror, especially when he did 

 not even suspect that he was infected. These worms eat the digesting 

 food floating in the intestine and sometimes bite the intestinal wall and 

 suck blood. They may become so numerous as to block the intestine 

 and thus cause death unless removed surgically. 



The life processes of ascaris show several advances over those of 

 planaria. The digestive system consists of a tube that runs the length 

 of the body with a mouth at one end and an anus at the other. The 

 advantage of this arrangement over that in which a single opening had 

 to serve as both a mouth and an anus has been discussed in the chapter 

 on planaria. There is a pharynx just back of the mouth which pumps 

 the digesting food through the intestine where absorption takes place. 

 There seem to be no digestive glands present to secrete enzymes of diges- 



