NEMATHELMIA. 



47 



CHAPTER IV. NEMATHELMIA (Thread-worms). 

 7. ASCAEJS (The Round- worm). 



THE large group of Nemathelmia or Thread-worms, includes both 

 free and endoparasitic forms. The latter are by far the more 

 numerous, and of these the most convenient type-genus is Ascaris 

 (the round-worm), one species of which A. lumbricoides infests 

 the small intestine of the human subject, while the much larger 

 A. megalocephala commonly occurs in the intestines of the horse. 

 The following description applies to both species, but when 

 measurements are given they refer to A. lumbricoides. 



MORPHOLOGY AND 

 PHYSIOLOGY. 



1. External Characters (Figs. 1 4 

 and 15). The bilaterally symmet- 

 rical body is cylindrical and tapers 

 to a blunt point at each end. The 

 dorsal and ventral surfaces are 

 much alike. Thread -worms are 

 not, as a rule, hermaphrodite, and 

 in Ascaris the two sexes are readily 

 distinguishable, for the male is 

 not only smaller than the female 

 ( $ about 6 inches, $ about 8 

 inches long), but the posterior end 

 of its body is sharply bent up on 

 the ventral side (Fig. 15). 



The colour is whitish, and in 

 the fresh condition the worm is 

 somewhat translucent, there being, 

 however, four more opaque longi- 

 tudinal streaks, named from their 

 relative positions the dorsal, ven- 

 tral, and lateral lines. The mouth, 

 situated at the anterior end, is 

 guarded by three projecting lips, one of which is dorsal, while 

 the other two meet together in the mid-ventral line. The gut 

 lias also a posterior opening, readily seen as a ventral slit, not 



Fig. 14 ASCARIS. a, Female 

 specimen of A. lumbricoides; 

 b, head ; c, egg. From v. Jaksch 

 (Clin. Diagnosis). 



