186 GENERAL ZOOLOGY 



month. Recent investigations indicate that Ascaris may 

 perhaps pass through larval stages in rats or mice before 

 entering man. 



Comparing Ascaris with a flat worm, it shows two great 

 structural advances a body-cavity and an anus. The 

 flatworm must ingest and egest through a single opening; 

 the round-worm has a much more convenient arrangement 

 with an entrance for food and an exit for faeces. The body 

 cavity is not a true ccelom, like that in the earthworm (Figs. 

 16, 89) and other animals, but is sufficiently developed to 

 make the general structure of Ascaris' body a tube-within- 

 a-tube, as described in Chapter II. It is simply a space 

 between the muscles of the body wall (Fig. 76, B, ra) and 

 the digestive tube. This space is a great convenience, for 

 it allows fluids to flow readily from one part of the body to 

 another without having to pass from cell to cell, as is the 

 case in sponges, ccelenterates, and also to some extent in 

 flatworms. 



OTHER NEMATODES 



An Ascaris living in a man usually causes no very serious 

 trouble, but sometimes one will crawl up the bile ducts into 

 the liver and give rise to an abscess, or lodge in the appendix 

 and induce appendicitis. Some other round-worms which 

 live in man are much more injurious. The hook-worm, 

 Necator americanus, is prevalent throughout the south- 

 eastern United States, where it was probably introduced 

 by slaves brought from Africa. It gnaws the inner wall of 

 the intestine and secretes toxins which impoverish the 

 blood and thus weaken the body. The hook-worm has a 

 peculiar life history the adults lay eggs in the intestine; 

 the eggs hatch into larvae which live for a time in excre- 

 ment or in the soil, and when grown enter man by boring 

 through the skin; they then get into the blood and finally 

 gain entrance to the alimentary canal by way of the lungs, 

 trachea, and larynx. Persons with a heavy hook-worm in- 

 fection may be abnormal in many ways: stunted and weak; 



