Chap. 26 ROUNDWORMS THE TUBULAR PLAN 521 



excreta. Under favoring conditions of temperature, moisture, and air the active 

 embryos develop in about two weeks. In another week, while they are still in 

 the shell, the minute worms molt and become active larvae. They are now 

 capable of infecting a host. When the eggs are swallowed, often on uncooked 

 vegetables, the larvae hatch in the small intestine. After repeated investiga- 

 tions upon animals which harbor the parasites for a time, it has been discov- 

 ered that the larvae do not continue to develop in the intestine. Instead, they 

 pierce the intestinal lining and enter the blood stream thus reaching succes- 

 sively the liver, heart, and lungs. They burrow out of the lungs, reach the 

 trachea and esophagus, and finally the intestine. This journey takes about ten 

 days during which the larvae increase from microscopic size to a length easily 

 visible to the naked eye. In the intestine they grow to maturity, six to 12 inches 

 long, the females larger than the males (Fig. 26.2). The average length of 

 their mature life in the intestine is about a year. The number of eggs in the 

 mature female may reach 27,000,000, probably more. 



Knowledge of the life cycle of this species of Ascaris and the successful treat- 

 ment of its human host are among the thousands of benefits to human life that 

 have come from experimentation upon animals. These parasites have not lived 

 out their life cycle in any animals which have been experimentally infected 

 with them. Yet, the larvae will migrate through the body in mice and guinea 

 pigs as well as in their human host. And this was the hardest part of their life 

 story to discover — why and how they take their roundabout route away from 

 the intestine through membranes and passageways and back to the intestine 

 again. 



General Structure. Nematodes are clothed with a tough, usually trans- 

 parent cuticle secreted by a layer of protoplasm in which there are nuclei but 

 no cell membranes (syncytium). Beneath the syncytium a layer of longitudinal 

 muscles is divided into four bands that extend the whole length of the body. 

 When the dorsal band contracts the ventral one is stretched and vice versa; 

 likewise, when the right side of the body is contracted the left side is stretched 

 and vice versa. The action of these muscles and probably some rebound 

 from the bent cuticle compose the entire locomotor outfit of nematodes. 

 It is responsible for their thrashing gait, a swinging whip in one direction, 

 and backward whip in the opposite. Even so, they make good progress when 

 they can push against particles of soil, or against food in the intestine, or as 

 they squirm through tissues. Water gives them little help. On a microscopic 

 slide a group of flexing nematodes might be taking a gymnastic exercise, much 

 bending and no locomotion. 



Between the muscles and the digestive tube there is considerable space, a 

 body cavity in that it holds the organs. However, it is not lined with epithelium, 

 and thus not a true body cavity or coelom comparable to that of the earth- 

 worm and of higher animals. 



