THE TUBE-WITHIN-A-TUBE BODY PLAN 



177 



todes tend to remain with a specific host, 

 although they are a httle more careless in 

 this regard than some. Occasionally they 

 attack a variety of hosts and may produce a 

 serious disease when they enter a new one. 

 For example, a hog may be riddled with 

 Trichmella with no apparent damage, 

 whereas a man with a similar infection is apt 

 to die because he is the "accidental host" 

 while the hog is the normal host. The hog 

 has had trichinella in its tissues so long 

 that it has built up some resistance to the 

 parasite. Since man gets the parasite only 

 occasionally, he has not developed any re- 

 sistance. Let us consider several common 

 roundworm parasites. 



Ascaris liimhricoides (Fig. 10-2) is a 

 common intestinal parasite of many domes- 

 tic animals as well as man himself. It is an 

 excellent example of the usual life cycle of 

 parasitic roundworms, although there are 

 wide modifications, as will be seen in trich- 

 inella, for example. It is not infrequently 

 found in the digestive tract of children, 

 since they are apt to get ascaris eggs on 

 their hands from the soil and transfer them 

 to the mouth ( Fig. 10-3 ) . The embryonated 

 eggs pass through the stomach to the intes- 

 tine where they hatch into tiny worms ( 0.2- 

 0.3 mm. long). These bore through the 

 intestinal wall into the lymph, then the 

 capillaries, and finally the general circula- 

 tion. They are carried through the heart to 

 the lungs where they grow somewhat in 

 size. Eventually the larvae break through 

 into the air sacs, migrate up the trachea, 

 and are swallowed, arriving in the intestine 

 for the second time. Here they mature, cop- 

 ulate, and lay eggs which pass out of the 

 host with the feces. The eggs may be picked 

 up directly by another host, or they may be- 

 come desiccated and blow around in the 

 dust to be engulfed at some later stage. 



In general, small numbers of ascarids are 

 relatively harmless, but large numbers can 

 cause serious illness. Sometimes they wan- 

 der away from their usual haunts: they 

 may crowd into the appendix or perforate 



Fig. 10-2. One of the largest round worm parasites 

 found in the intestine of both man and the pig as 

 well as other animals is Ascaris /ombr/coides. The 

 male is slightly smaller than the female and it pos- 

 sesses a curved posterior end. The female is about 

 25 cm. long. 



the gut wall, causing peritonitis; they may 

 even get into the nasal chambers, obstruct- 

 ing the air passages when full grown. When 

 large numbers of larvae move through the 

 lung tissue, they are apt to leave lesions 

 which may give pneumonia an opportunity 

 to gain a foothold. Appropriate vermifuges 

 can be used to remove this parasite. 



The adult ascaris probably maintains its 

 place in the intestine by active movements, 

 since it does not possess an attachment or- 

 gan such as the flukes and tapeworms do. 

 It feeds on the food in the gut by a pump- 

 ing action of its bulb-like pharynx. To keep 

 from being digested by the enzymes se- 

 creted by tiie host, ascaris, like all intestinal 

 parasites, is protected by a tough cuticle, 

 through which probably is secreted sub- 

 stances that counteract the hydrolizing 



