154 MANUAL OF ZOOLOGY SECT, vi 



the axis of the tube and having the sperm-cells attached 

 to it; still further back the sperms become gradually 

 differentiated, and are finally set free in the vas deferens. 



The organs of the female (Fig. 76) resemble those of the 

 male, but are double instead of single. There are two 

 coiled, thread-like ovaries (ovy*\ each passing insensibly 

 into a uterus (ut.}. In the Ovary, as in the testis, the eggs 

 are developed in connection with an axial cord or rachis. 

 The two uteri unite in a short muscular vagina (vag.) which 

 opens, as already seen, on the ventral surface of the body 

 (gnp.) at about one-third of the entire length from the head. 



The Nematodes in general vary greatly in size, from about 

 i mm. or less to 2 metres (6 feet) in the case of the Guinea- 

 worm, the length being always great in proportion to the 

 diameter, and the body being always bluntly pointed at the 

 anterior end and either pointed or forked posteriorly. 



The mouth is frequently armed with spines by means of 

 which the worms draw blood from the intestinal blood- 

 vessels of their host. Many free-living forms have a sharp 

 stylet for piercing the tissues of the plants on which they 

 feed, and a suctorial apparatus for absorbing their juices. 



The nervous system has in most the same general struc- 

 ture as in Ascaris, and the same holds good of the repro- 

 ductive apparatus, A few are hermaphrodite ; but, instead 

 of a double set of reproductive organs as in Platyhelminthes, 

 they have organs similar to those of the female Ascaris, the 

 gonads producing first sperms and afterwards ova. 



One of the most terrible parasites of man is a Nematode 

 called Trichina spiralis (Fig. 79), a minute worm, the male 

 (C) a little over i mm. (^ inch) in length, the female (B) 

 about 3 mm. (J inch). In the adult or sexual condition it 

 lives in the intestine of man, the pig, and other mammals. 

 Internal impregnation takes place, the eggs develop in the 



