154 



COLLEGE ZOOLOGY 



Esophagus Body cavity (pseudocoel) 



Seminal receptacle 

 Recfum 



Moufh Esophageal Infesfine Egg Uferus Developing Vulva 

 bulb larva 



Figure 80. Internal structure of the free-living vinegar eel. Parts of digestive and reproductive 

 systems are shown. Natural size 2 mm. in length. 



cases even their species could be determined 

 by an examination of their erstwhile nema- 

 tode parasites" (Cobb). 



Parasitic plant nematodes 



Nematodes live in or on many different 

 kinds of plants, causing enormous economic 

 loss. More than 1000 different species of 

 nematodes are known to attack plants. The 

 damage these tiny parasites do on American 

 farms amounts to $500,000,000 a year. The 

 common garden roundworm Meloidogyne 

 lives in roots of over 1700 species of plants. 

 The worms lay their eggs either directly in 

 the roots or in the nearby soil. Certain 

 nematodes stimulate plant tissue to form 

 knotlike galls on the roots. Others enter 

 leaves and move about eating the contents 

 of the cells. Some worms stay on the surface 

 of the plant, bury their heads into the tis- 

 sue, and suck out the juice. These parasites 

 sap the plant's vigor, open the way for bac- 

 teria and fungi, and injure growing points. 

 The only means of control of these plant 

 parasites is by crop rotation, soil sterilization, 

 and development of resistant varieties of 

 plants. A lima bean has recently been de- 

 veloped by scientists which is nematode re- 

 sistant. 



Other parasitic roundworms 



Among the representative roundworms 

 that may be found in other vertebrate ani- 

 mals are the cecum worm of chickens, the 

 dog ascarid, the horse roundworm, and 

 horsehair worms. 



The dog ascarid 



Toxocara canis is especially prevalent in 

 puppies which became infected through the 

 placenta. Dogs become infected by swallow- 

 ing the eggs. The larvae migrate through the 

 body as do those of Ascaris in man. Dogs 

 acquire an immunity as a result of the infec- 

 tion; and after three or four months, the 

 worms are cast out and susceptibility to fur- 

 ther infection is greatly reduced. 



The cecum worm of chickens 



Heterakis gallinae (Fig. 81) lays eggs, 

 which are passed in the feces of infected 

 birds and are swallowed by other birds; the 

 young that hatch from these eggs in the 

 small intestine move on into the ceca. They 

 do not seriously injure the fowls but are 

 of great economic importance since they 

 carry with them a protozoan parasite, His- 

 tomonas meleagridis, which is the causative 

 agent of the disease of turkeys known as 

 blackhead. 



The horse roundworm 



Horse strong}'le (Strongylus vulgaris) is 

 world-wide in distribution but especially 

 prevalent in warm countries. It lives in the 

 cecum or colon, attached by the mouth to 

 the mucosa, from which it sucks blood. Loss 

 of blood results in anemia. The eggs of 

 Strongylus are deposited in the feces, where 

 they give rise to infective larvae. These, 

 when ingested by a horse, migrate to the 

 posterior mesenteric artery, where an aneu- 

 rysm may be produced; they then move on 

 to the cecum where they become encysted 

 in the submucosa; and finally they break out 



