Animals P by: Geoffrey LaPage Published by ... - PSSurvival.com

------ -
Animals Parasitic in Man --. 1 __~
___~_--- --~-~ -_-~---
also includes certain species of~;~pion&%d
~--- spider%-whichmayinSicton~&an ---
dangerous bites or stings.
DIRECT AND INDIRECT’LIFE HISTORIES I
The anatomy of all these different kinds of animals cannot be
fully described in a book of this size, but succeeding chapters
will give, as the mode of life of each parasitic species is described,
the anatomical ‘features that are necessary for a clear
understanding of the parasitic associations that these species
have established with man and their other hosts. Before we go
on to consider these species, however, an important general
feature of their life histories must be explained.
The term life history is given to the whole series of changes
of structure and adaptation experienced by any animal,
whether it is parasitic or not, between the moment when its
life begins in the fertilized egg, or, among the single-celled
Protozoa, the equivalent of the fertilized egg, and the time
when it reaches the phase of the life history which, in its turn,
produces fertilized eggs or their equivalent. This final stage is
called the adult stage, although among the single-celled Protozoa
it may be given different names. The life history of a
butterfly, for instance, begins with the fertilized egg laid by
the female butterfly, from which a young caterpillar (larva)
hatches out. The caterpillar feeds and grows and eventually
changes into a chrysalis (pupa). Inside the pupa the adult
butterfly is formed and this lays more fertilized eggs, so that
the life history begins again.
In a similar way the life histories of parasitic animals include
a series of changes of structure which end with an adult
stage, or, among the Protozoa, its equivalent; and it is necessary
to know all these different phases of the life histories of
all the different species of parasitic animals, because they are
all closely associated with, and adapted to, the parasitic animal’s
need to establish and maintain a parasitic association
with its hosts. Another reason for knowing all these phases is
the fact that knowledge of them enables us to understand
better how each parasitic animal gets from one host to another,
28