xii INTRODUCTION 
mouth and nose. In other words, they are ecto- 
parasites, or, as they are sometimes called, Epizoa. 
Fixed parasites are chiefly endo-parasites, or Entozoa 
—that is, they live in those cavities which communicate 
with the exterior, such as the intestines, lungs, and 
bronchi, or sometimes in closed cavities, as the blood- 
vessels or peritoneal sacs. The endo- parasites are 
degenerate, but possess great vitality and enormous 
fertility, and are often hermaphrodite. 
The majority of the Entozoa require more than one 
host in which to pass their life-history, and these are 
usually of different species. The host in which they 
reach the adult reproductive stage is spoken of as the 
“ultimate’’ or ‘‘ definitive host,’ while the former 
host is known as the “intermediate host.’’ The cycle 
commences with ova or larve which have gained the 
exterior. These invade the ‘‘ intermediate host,’ within 
which they undergo certain changes in their develop- 
ment, and are then taken in by the “ ultimate host,” in 
which they complete their process of development and 
reach the adult stage. 
As a general rule each mammal has its own particular 
parasites, but some of the ecto-parasites are capable of 
existence on several animals. Hence, the mange para- 
sites of horses may affect man, but their vitality is 
thereby greatly impaired, and they are unable to exist 
any length of time away from their natural host. 
Entozoa may occasionally invade a host of a different 
species. For instance, Trichina of the pig sometimes 
invades the muscles of man, while Bothriocephalus latus, 
a tapeworm found in man, in some countries can live in 
the dog. 
Etiology. — The old theory which was supposed to 
account for the presence of parasites was that of spon- 
taneous generation, but with the advancement of science 
and research, this theory has been discarded. The great 
