GENERAL ZOOLOGY 



as previously indicated, the intestinal epithelium of the nematode stores 

 food reserves and transfers nutrients into the fluid of the pseudocoel, for dis- 

 tribution to all parts of the worm. 



In Ascans, cellular metabolism differs from the overall pattern found in 

 most other animals because free oxygen is almost completely lacking in the 

 intestine of the host. The release of energy in the cells of the ascaris occurs 

 almost entirely by an anaerobic process, which in all its details is com- 

 parable to the anaerobic phase of cellular metabolism (pp. 36 37). 

 Linked enzyme systems are present which in the absence of atmospheric 

 oxvgen are capable of partially oxidizing energy-rich carbohydrate mole- 

 cules, such as glycogen, by decomposing them into various organic acids. 

 This is a less efficient mode of energy release than the aerobic cycle; it has 

 been imposed on the parasite by the conditions of its environment. 



The reproductive organs of the ascaris are simple tubes, closed at their 

 inner ends and lying free in the pseudocoel except for their attachments 

 at the external openings. The posterior portions of the paired oviducts, in 

 the female, expand to form the so-called uteri, which unite at the vagina. 

 The male reproductive system in this species, as in many other nematodes, is 

 unpaired (Fig. 12.4). 



Spermatozoa, which in the ascaris are not flagellated but amoeboid, are 

 introduced into the vagina at copulation. Ova are fertilized in the oviducts, 

 and the resultant zygotes become surrounded by resistant proteinaceous shells 

 secreted by oviducal glands. These eggs are released into the intestinal 

 cavity of the host and reach the exterior with the host's feces. Development 

 of the embryo to an infective juvenile stage occurs within the shell of the 

 egg, at a rate which depends on external conditions. The young worms do 

 not hatch until the eggs have been ingested by the proper host. When the 

 juvenile thus reaches the intestine of man, it emerges, penetrates the intestinal 

 mucosa, and reaches the circulatory system. It is carried by the blood through 

 the liver and heart and to the lungs, where it enters an alveolus by rupturing 

 a capillary. After molting and growing, the worm makes its way up the 

 trachea and into the esophagus, eventually returning to the intestine as a 

 small adult. Full growth and sexual maturity are finally attained after 

 some time. 



Infestation of man by Ascans lumbncoides is not uncommon and is sometimes 

 serious; the migrations of the juvenile worms through the body can give rise 

 to widespread complications. Consideration of the mode of transfer from host 

 to host shows clearly, however, that under modern conditions of sanitation 

 infestation with A. lumbncoides is certainly avoidable. 



Many other species of nematodes are dangerous parasites of man and do- 

 mestic animals in different regions. For example, the hookworms, such as 

 the American form, Necalor amencanus, and the related Old World hookworm, 

 Ancylostoma duodenale, are intestinal parasites of man which in this country 

 are more or less limited to warmer and more humid regions. Hookworms are 

 armed with hook-like buccal plates with which they erode the intestinal lining 



352 



