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

Animals Parasitic in Man
Plate 4a) some of which are parasitic in man. Many of the
Nematoda are not parasitic; they live in the soil, in the sea,
in freshwater, in almost any situation, in fact, that provides_-
them with sufficient moisture; but some of them are parasitic
and these may cause serious diseases of man and other
animals. Nematodes are cylindrical, smooth-skinned worms,
which do not show on the surfaces of their bodies the concentric
rings that all the Annelida show. Frequently they
are pointed at both ends. Their bodies are built on the plan
of two tubes, one of which, the food canal, is inside the
other, which is made up of the muscles and the skin outside
these. A mouth at the anterior end opens into the food
canal, which frequently has, behind the mouth, a muscular
pharynx, which acts as a sucking pump with which the food
is sucked in; behind the pharynx the food canal runs as a
straight tube to the vent (anus), which is usually just in
front of the posterior end of the worm. Between the food
canal and the muscles and skin there is fluid under pressure
which gives the body form and provides a resistance against
which the muscles act. Nematodes are usually unisexual;
the sexes are, that is to say, in separate individuals, the
males being usually smaller than the females.
4. The IWyhelminthes or Flatworms. This phylum includes
the e$dyworms, which are not parasitic; and the flukes
( Plate la) and tapeworms (Plate 1 b), all of which are para-
S%G. The tiies of flatworms are flattened so that the back
c’ames near to the underside and the whole body is- solid, the
interior being filled with a spongy tissue called the parenchymu,
in which the various organs are embedded. The parasitic
species (flukes zld tapeworms) have hooks, or suckers,
cr both, with which they hold on to their hosts. Most of the
flatworms are hermaphrodite? the male and female sexual
organs b&g in the same individual.
6. The ArthrGpoda, This phylum includes the Class &&acea,
to which the lobsters, shrimps, crabs, woodlice, and their
numerous relatives belong; the Class Myiaoda, which includes
the centipedes and millipedes; the Class Insecta, examples
of which are the mosquitoes, tsetse flies, house flies,
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