NEMATODE WORMS. 57 



serve sense-organs in the form of simple eyes in those forms 

 that are free-living, but not in the parasitic species. In some, 

 such as the Acanthocephala, the mouth and alimentary canal is 

 entirely absent, whilst in the Nematodes a true digestive system 

 is developed. Many of the Nemathelminthes develop without 

 any metamorphosis. Where larval forms occur the larvae are 

 often found in different hosts, and in any case in different pr.its 

 of the same host. 



Nematoda. 



The round-worms which we have especially to consider in 

 this work are the Nematodes or Thread-worms, which include 

 various parasites in animals and birds, as well as in man, and 

 also parasites that attack plants. 



The body of Nematodes is thin and thread-like, hence their 

 popular name " Thread-worms.'' The mouth leads into an 

 oesophagus which has thick muscular walls and often a chitinous 

 lining, and is frequently dilated behind into a muscular bulb, 

 the pharynx. The intestine following the pharynx opens by a 

 posterior pore, the anus, on the ventral surface. In a transverse 

 section of a nematode the muscle is seen to be divided into four 

 areas, broken at the sides by the two lateral lines in which run 

 the excretory canals, and above and below by a dorsal and 

 ventral line. Nematodes are always of separate sexes, never 

 hermaphrodite as in the Flat-worms, except in one or two 

 notable exceptions, in which first male cells — spermatozoa — and 

 then female cells — ova — are formed in the same individual. 

 Male nematodes are always smaller than the females, and have 

 the posterior end of the body curved ; moreover, they are usu- 

 ally provided with a pair of recurved hooks for attachment to 

 the female (fig. 18, d). Nematodes increase by true sexual 

 reproduction. 



Sexual reproduction consists of the union of a male cell 

 which is known as a sperm cell or spermatozoo7i with the female 

 or ijenn cell ,■ the result of the fertilisation — that is, the entry of 



