CHAPTER VI 

 NEMATHELMINTHES. ROUND-WORMS 



These are the round or thread-worms, which are characterized 

 by their long, slender, cylindrical bodies that are entirely devoid of 

 internal or external segmentation. Their internal structure is usually 

 moderately simple, as might, perhaps, be expected of a group, many 

 members of which are parasitic. Some of them are of microscopic 

 size; others reach a length of a meter. Their chief if not only claim 

 to economic importance lies in their parasitic habits. 



FIG. 22. Parts of Ascaris lumbricoides. a,-b, and c, enlarged; d, highly magnified 

 a, posterior end of a male with two penial setae (Sp); b, anterior end from the 

 dorsal side, showing the dorsal lip with its two papillae; c, the same from the ventral 

 side with the two lateral ventral lips and the excretory pore (P) ; d, egg with external 

 membrane of small clear spherules. (From Hegner, College Zoology, after Sedgwick, 

 from Leuckart.) 



One of the larger and more common of the round-worms is the 

 genus Ascaris, Figs. 22, 23 and 24, found as a parasite in the intestine 

 of man, horses and pigs. It is a brownish worm, about 6 mm. in 

 diameter and from 12 to 30 cm. long, the female being usually some- 

 what the larger; the main features of its anatomy are illustrated by the 

 accompanying figures. The species common to man is A. lumbricoides, 

 the human eel worm. It is one of the most common of the human 

 worm parasites, being frequently found in children from 5 to 10 

 years of age, and in women more often than in men. It is also 

 more common in warm than in cold climates, and is more frequently 



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