11 



ROUND W^ O R M S 



Besides the flukes and tapeworms there is another group of 

 parasitic worms, even more important medically and economi- 

 cally, and of at least equal biological interest. This is the group 



Nematoda, or round- 

 worms, so called because 

 in contrast to the flat- 

 worms their body is 

 circular in cross-section, 

 and so resembles a thread. 



ASCARIS 



Some of the largest 

 roundworms are those 

 belonging to the genus 

 Ascaris. The species suillce 

 is frequently present in the 

 small intestine of pigs, 

 and lumhricoides (Fig. 97) 

 is not uncommon in man, 

 especially in children ; the 

 male may be as large as 17 

 centimetres long and 0-2 

 centimetres in diameter, 

 while the female may be 

 25 centimetres by 0-5. 

 A. megalocephala, which lives in the horse, is even larger. The 

 following description is intended primarily for the human form, 

 but will apply almost equally well to the others, which are indeed, 

 according to some workers, merely physiological races of the 

 human species. 



The general shape is that of a cylinder with pointed ends, 

 and the surface is smooth and yellowish-white in colour. Along 

 the middle of the back and of the ventral side run white lines, 

 and there is a brownish line along the middle of each flank. At 



134 



Fig. 97. — Ascaris lumbricoides 



A, Male; B, Female. 



a. Anterior end ; an., anus ; e., 'excretory' pore; 

 genital opening ; p., posterior end p.s., penial setae 



g.o. 



