PARASITES OF ANIMALS. 57 
ally more and more like its grandparent. It finally fills up 
the whole interior of the body of the enclosing embryo, sexual 
organs appear, and then the outer skin of the latter is cast off. 
Thus the young Echinorhynchus is formed, but in order to be- 
come mature it must be swallowed, with its crustacean host, 
by some fish. In the intestine of the fish it becomes sexually 
mature in about a week. 
The giant Echinorhynchus of the hog (E. gigas), Figure 74,” 
is the only species known to infest domestic quadrupeds. 
TV. Nematopes. (Round-worms). - 
This order includes a great number of worms, which occur 
‘in all classes of animals, both vertebrates and invertebrates, 
together with some that are not parasitic, but live in water, 
moist earth, or decaying animal or vegetable matters. They 
are much more highly organized than those of the preceding’ 
orders. The pin-worms of children, the common round-worms 
of man and domestic animals, and the notorious Trichina spi 
ralis are examples of this order. (See figures 76 to 88.) 
These worms are almost without exception long, round, and 
rather slender, varying in length from a tenth of an inch or 
less to three feet, or even more, and usually tapering to one or 
both ends. The external surface of the body is generally 
smooth and uniform, without suckers or other appendages. In 
the interior of the body there is a continuous visceral cavity, 
containing a distinct digestive system, theintestine generally 
extending from one end of the body to the other, and-well de- 
veloped reproductive organs, the sexes being separated. The 
mouth is usually at the smaller end of the body and is often 
surrounded by papille or other organs. Some of the species 
are viviparous, like Zrichina ; others produce eggs, like the com- 
mon round-worms and pin-worms. There is great diversity 
in their habits and modes of development. Some of them 
pass through alternate generations and remarkable trans- 
formations, living in some cases, like Trichina, in two different 
animals in the larval and adult states, in other cases pass- 
ing the young states in the water, as free worms, and after- 
wards becoming parasitic, when swallowed by the proper ani- 
mal, in which alone they develop their reproductive organs 
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