Mouf/7 



-^ ^5p/cu/es 



CHAPTER XXXIII 

 PHYLUM NEMATHELMINTHES 



The representatives of the phylum Nemathelminthes (n?m a thel 

 min' thez; G., nematos, thread, and helminthos, worm) are called, collec- 

 tively, roundworms or threadworms. Some of them are free-living; 

 others are found in plant tissues, where they may be the cause of plant 



diseases; and still others are parasites in 

 other animals. Generally speaking, the 

 nemathelminths are small and even micro- 

 £xcre-/vr ^copic, but a small number reach a larger 

 pore size and a few may even be several inches in 

 length. 



206. Structure of an Ascaris. — An ascaris 

 may be taken as a type of the phylum. 

 Logically a free-Uving type would be pref- 

 erable, but the size of an ascaris and the 

 ease with which it may be secured render it 

 the most practical one. The genus Ascaris 

 includes roundworms of about the size and 

 with the general proportions of an earthworm 

 but possessing no metamerism (Fig. 88). A 

 common species, known as the eelworm or 

 the pig ascaris, Ascaris lumhricoides hinnsieus, 

 is found in the intestines of pigs and human 

 beings. The ascaris found in man and in 

 pigs are morphologically identical but physi- 

 ,. . 1,. V 1/ '^R^ ologically different. Because of certain phys- 



Linnaeus. A, male. X ,^t. o> => '^ . . ~ 



the anterior end of the body, c, iological differences, some authorities prefer 

 Jhowi^Hhrl hpl "a p^steno; the name Ascans suum for the pig ascaris and 

 end. (S to D from Leuckart, AscaHs Iwmhrico ides for the ascaris of man. 



"Para^itendesMenschen.") Mag- ^^^ ^^^^j^ .^ ^^ ^^^ anterior end of the 

 nified. 



body and an anal opening near the posterior 

 end. When opened, the animal is seen to possess a relatively thin body 

 wall surrounding a central cavity, through which runs the alimentary 

 canal (Fig. 89). This canal consists of a short muscular sucking pharynx, 

 a long nonmuscular intestine, and a short rectum. The excretory system 

 consists of two longitudinal canals, one of which runs along each side of 

 the body, the two opening to the outside by a single pore on the ventral 



178 



Fig. 88. — Ascaris lumhricoides 



X H- 



