CHAPTER XXXIII 



PHYLUM NEMATHELMINTHES 



Excre-hsry 

 pore 



Moufh 



^ 5p/ca/es 



The representatives of the phylum Nemathelminthes (nem 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 

 nemat helminths are small and even micro- 

 scopic, but a small number reach a larger 

 size and a few may even be several inches in 

 length. 



204. Structure of an Ascaris. — An ascaris 

 may be taken as a type of the phylum. 

 Logically a free-living 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. 82). A 

 common species, known as the eelworm or 

 the pig ascaris, Ascom lu^nbricoidesljinnaeus, 

 is found in the intestines of pigs and human 

 beings. The mouth is at the anterior end of 

 the body and an anal opening at or near the 

 posterior end. When opened, the animal is unn 

 seen to possess a relatively thin body wall sur- the anterior end of the body. c. 



,. J , •, .1 11-1 end view of the anterior end, 



roundmg a central cavity, through which runs showing three lips. D, posterior 

 the alimentary canal (Fig 83). This canal end. (B to D from Leuckart, 



. , p 1 , , , . 1 "Parasitendes Menschen") Mag- 



consists of a short muscular sucking pharynx, njfied. 

 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 surface near the mouth. 

 There are no flame cells, but four large branched excretory cells are 

 present in the anterior part of the body. A circumpharyngeal nerve ring, 

 containing nerve cells, surrounds the pharynx and is connected with two 



175 



Fig. 82.- 



Ascaris lumbricoides 



aeus. A, male. X }4- B, 



