128 ZOOLOGY 
The testis opens into a vesicula seminalis of a much 
greater diameter than the testis tube, its lining epithelium 
throws out processes into the lumen which resemble pseudo- 
podia. These possibly perform the same function as the cilia 
which usually occur in the vesicula seminalis of other animals, 
cilia being entirely unknown in Nematodes. 
The vesicula seminalis opens into a ductus ejaculatorius, 
with muscular walls. This ductus opens on the dorsal side 
of the rectum. On each side of it is a sac containing a 
chitinous spicule which is protrusible, and is doubtless used 
in copulation. 
The female organs are double, and consist of ovaries, 
oviducts, uteri, and a vagina, the latter opening on the 
ventral middle line. The ovary consists of a tube in which 
the egg cells are formed in enormous numbers as stalked 
structures borne on a rachis. The oviduct differs from the 
ovary only in containing free ova, it leads on each side into a 
uterus in which numerous spermatozoa are found, and where 
the fertilisation of the ovum takes place. The two uteri 
unite, and open by means of a short vagina to the exterior. 
The eggs are laid in millions, each surrounded by a smooth 
shell. The embryos develope in water or in damp earth, and 
are probably introduced into their human host by the drinking 
of dirty water. 
The majority of Nematodes are parasitic, at any rate during 
a portion of their life, but a good many lead a free existence in 
damp earth, moss, and decaying matter, or in salt or fresh 
water. These are mostly minute forms, and are capable of 
withstanding a considerable amount of desiccation. The free- 
living forms, with certain others that inhabit plants, are 
included in the family Anguillulidae. 
Tylenchus tritici does great damage to corn crops, its 
presence leads to the grains of corn being replaced by a dark 
brown gall. Inside this gall a small cluster of these minute 
worms are found. When one of the galls is sown with the 
seed, and rain follows, the parasites leave the gall and infest 
the young plants. The parasites pair within the gall on the 
corn ear, and eggs are laid within the gall. 
Amongst the Nematodes which are parasitic in animals, 
