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CHAPTER VIII. 

 NEMATHELMINTHS. 



Nemat helminths are round, unsegmented worms, as 

 a rule tapering at both ends. There is a body-cavity, but 

 without any epithelial lining. Sensory organs, such as 

 papillae in the neighbourhood of the mouth, and genital 

 openings are present, and eye-spots may be found in 

 allied free-living species. The nervous system varies, but 

 in all there is an anterior and a posterior nerve ring with 

 longitudinal dorsal and ventral nerve trunks. These 

 trunks have lateral branches which may form complete 

 rings. 



In most species there is a complete alimentary canal, 

 with mouth, intestine, and anus. 



The sexes are distinct : the male organ, always a single 

 testis, opens into the rectum, forming a cloaca, and there 

 are copulatory spicules. The female organs, usually two 

 ovaries, unite into a single tube vagina which opens 

 separately from the alimentary canal. 



The nemathelminths are often parasitic during a 

 portion of their lives only. Some are parasitic through- 

 out the whole of their existence, and in such cases the 

 larval and adult, sexual, stages are frequently passed 

 in different hosts. There are three classes : 



(1) Ncmatoda. With mouth and alimentary canal ; 

 longitudinal unstriped muscles only are found. There is 

 a single testicle and vas deferens which opens into the 

 rectum. Important parasites of man and the domesticated 

 animals belong to this class. 



(2) Nematomorpha are parasitic in their larval stages in 

 aquatic larvae, insects, &c., and in their adult stage are 



