ROUNDWORMS— THE NEMATHELMINTHES 



157 



Excretion is carried out by a pair of excretory tubes that run the 

 length of the body on either side and unite and empty through a pore at 

 the anterior end a short distance behind the mouth. The muscular sys- 

 tem shows no noteworthy changes in comparison with planaria. 



The nervous system consists of a nerve ring which circles the 

 pharynx and two nerve cords running the length of the body as in 

 planaria, but these two are dorsal and ventral rather than lateral. Con- 

 necting nerves circle the intestine at intervals and smaller branches are 

 given out to all parts of the body. 



Intestine 



Excretory tube Pharynx 



Pseudocoel Uterus with Ovary 



embryos 



Vagina 



Mouth 



Excretory 

 Genital pore pore 



Mouth 



Dorsal nerve cord Pharynx 



N. Seminal vesicle 



Penial setae 



Testis Nerve ring 



Ventral nerve coral 



Fig. 11.3. Internal structure of ascaris ; female above and male below. The excretory 



system is shown only in the female and the nervous system is shown only in the male ; 



both are present in both sexes, of course. 



The reproductive system is represented by organs similar to those 

 found in planaria, but the one significant difference is the separation of 

 the sexes. There are definite male and female ascaris and they can be 

 recognized externally rather easily. The female is about a fourth of the 

 body length longer than the male and somewhat thicker, and the male 

 has a distinct crook at the posterior end with a pair of small hair-like 

 projections called penial setae near the end of the crook. Because the 

 sexes are distinct there must be at least two animals in a person's body 

 before the infection can be spread to others. The male and female copu- 

 late in the intestine, and the female soon becomes an egg-producing ma- 

 chine with a daily egg production that would make the world's champion 

 hen seem poor in comparison. Something like 200,000 eggs in twenty- 

 four hours is a good estimate for a healthy ascaris female. These pass 



