INORH 
00) 
ZOOLOG Y. 
35 
young Trichina is set free, and deposits eggs, the embryos 
from which bore through the viscera of the unfortunate 
one. The Trichina belongs to the family of thread-worms 
or Nematoda. Associated with the horse-hair and thread- 
worms is the Echinorhynchus, or Bristly-snout (Fig. 27), so 
called from the proboscis or snout (Fig 27, a), armed with re- 
curved hooks, being the most striking feature in its organiza- 
tion. The three families of Round Worms live within other 
animals,—the Trichina in the pig and man, the Echinorhyn- 
chus in the flounder, the Gordius, or horse-hair worm, in 
insects. They are sufficiently alike in their general organi- 
zation to be the descendant of a common ancestor ; their 
modifications are due to their different modes of life. 
Turning now to the Flat Worms, we see, according to Tree 
IIL, that the Turbellaria are probably the oldest of this 
group. They are arranged in two families, Dendroccela 
and Rhabdoccela, according as the intestine is branched, 
as in the Planaria (Fig. 28), or straight, as in the Prorhyn- 
chus (Fig. 29) or Vortex. The Planaria are found prin- 
cipally .in fresh water, but also in the sea, adhering to 
stones or stems of plants. In the haired covering of their 
vodies, in their internal organization and embryo forms, 
the Turbellaria are closely allied. to the Infusoria, from 
Which, most likely, they have been derived. From the 
Turbellaria have retfograded the Trematoda, of which the 
liver fluke (Distoma) (Fig. 30) is a common example. By 
retrograding, I mean that the Flukes have lost organs, 
from want of use, which they would have retained had 
they not lived as parasites in the viscera of other animals, 
The Flukes are found in almost every kind of animal. 
Imagine a long chain of Flukes living as one individual, 
and we have the Tape-worm, or Tzenia (Fig. 31), as a repre- 
sentative of the Cestoda. Either the Tape-worm has split 
into Flukes, or the Flukes have colonized and made the 
Tape-worm, or possibly they are both aborted Turbellarians. 
