CHAPTER VIII 
NEMATODA 
CHARACTERISTICS.— Animals with an elongated unsegmented body, 
tapering at each end. A well-developed cuticle is secreted by 
the epudermis. A digestive system is present, und the excretory 
system takes the form of lateral ducts which open antervorly 
by a median ventral pore. As a rule, they are dioecious, 
and many are endoparasitic ; among the parasitic forms an 
alternation of hermaphrodite and bisexwal generations may 
occur. A ciliated epithelium is universally absent. 
The Nematoda are colloquially known as thread-worms. 
The order contains a great number of species, many of them 
parasitic ; in fact, there are said to be as many species of para- 
sitic Nematodes as all the other endoparasites together. 
About twenty different species attack man, and they occur in 
almost every organ of the body, often inducing sufficient trouble 
to cause death. The free species are usually small, often 
microscopic. The parasitic forms are as a rule larger, the Guinea 
worm, Milaria medinensis, which lives in the subcutaneous 
tissues of men and horses in the tropics, attains a length of 
6 feet, and the female Hustrongylus gigas, which lives in the 
kidneys of mammals, may be 3 feet or more long. 
Ascaris lumbricoides inhabits the human intestine and 
stomach, and is not uncommon in children. It is a white 
cylindrical animal pointed at each end. The female measures 
from 9 to 14 inches in leneth. The male is about half as 
long; it is rarer than the female, and may be distinguished by 
its curved hinder end, and the presence of two bristles in the 
neighbourhood of the anus. 
The mouth is terminal and surrounded by three lips, a 
