58 BOARD OF AGRICULTURE. 
and become mature. Some species seem to have a much 
more simple history during development. There are but few 
species, however, of which the complete history is known. 
The Trichina spiralis is, perhaps, better understood in all 
stages of its life than any other species. Important discov- 
eries remain to be made, without doubt, concerning other 
species that infest our own bodies, as well as those of our 
“domestic animals. At present the complete histories of the 
two commonest human parasites, the round-worm and _ pin- 
worm, aresstill unknown, although they have bccn most care- 
fully investigated and experimented upon by many able nat- 
~ uralists. 
‘Of this order about 1,000 species have been already described, 
but it is probable that a considerablenumber of these will event- 
ually prove to be only the young of others. They particu- 
larly abound in mammals, birds, and fishes, inhabitine the in- 
testine, stomach, windpipe, lungs, kidneys, muscles, brain, and 
various other parts of the body. Twelve species or more are 
liable to infest human beings. 
ACARINA. 
In addition to the internal parasitic worms, there are certain 
parasites which belong to other groups. Among these are 
certain forms of acarina, or mites, some of which are, perhaps, 
only accidentally parasitic, like the Acarus dysenterie mentioned 
in our previous lecture. But the genus Pentastoma includes 
species which are true parasites and have nearly the same 
habits as some of the parasitic worms, which, indeed, they 
closely resemble in general appearance. Formerly these par- 
asites were regarded as worms by all authors, but they are 
now generally referred to the Acarina, although they differ 
greatly from the more typical forms. One of these species 
is represented in Figure 84. 
Protozoa. 
Several genera and species of infusoria have been found liv- 
ing in the human body, or in the excreta, especially of pa- 
tients attacked with certain contagious diseases. The effects 
of these parasites and their relations to diseases are not well 
understood. Digitized by Microsoft® 
