CHAPTER XVII 



THE NEMATODA. PARASITISM 



The size of an animal is no measure of its importance 

 in the economy of nature. All the elephants 

 Nemato a. an( j wna i eg t g e ther have far less effect on the 

 course of events in the world than is achieved by some 

 half-dozen kinds of Protozoa, or again by a few species of 

 parasitic worms. We have dealt already with some of 

 these little organisms. There remains one group of them 

 which now demands our attention. These are the Nema- 

 toda. The Arthropoda, to which the last two chapters 

 have been devoted, are the most active and conspicuous 

 of invertebrate animals. Utterly unlike them in most 

 respects, yet strangely allied to them in the broadest out- 

 lines of their morphology, the Nematodes are a group of 

 worm-like animals, mosily of small size and always of 

 retiring habits. One of the largest of them is Ascaris 

 lumbricoides, the Human Roundworm. 1 



Ascaris lumbricoides lives normally in the small intestine 

 of man. It is a yellowish-white worm, which 

 Ascaris. re aches a length of 25 centimetres in the female 



and 17 in the male, cylindrical, but tapering towards the 

 ends, and quite smooth. Along the middle of the back 

 and of the ventral side run dead white lines, and there is 

 a brownish line along the middle of each flank. At 

 the front end is the mouth, guarded by three lips, one 

 above and one at each side below. The dorsal lip bears 

 at its base two papilla and the ventro-lateral lips one each. 

 The edges of the lips are finely toothed. On the under 

 side, about two millimetres from the mouth, an excretory 

 1 A still larger species is A. megalocephala of the horse, to which the 

 following description will apply almost equally well. 



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