264 
COMPARATIVE ZOOLOGY. 
It has also close relations with the other subkingdoms of 
the bilaterally symmetrical animals. Through the Poly- 
zoa and Brachiopoda, it approaches the Mollusca; through 
the Annelides, the Arthropoda; and through other forms, 
the Tunicata, and so the Vertebrata. The subkingdom 
thus stands in the centre of several subkingdoms, with 
affinities towards all. Nor are indications of connection 
with Coelenterata and Echinodermata wanting. 
The Vermes are bilaterally symmetrical animals,with one 
or many segments, no jointed legs. They usually have a soft 
skin, and peculiar excretory organs—the segmental organs. 
Many of the Worms are parasitic, and most of the en- 
doparasites belong to this group. 
There are numerous classes, of which only the most im¬ 
portant are mentioned. 
Class I.— Platyhelminthes. 
The Flat-worms 
include some free 
forms, as the Plana- 
ria, common in fresh 
water, and the Tape¬ 
worms and Flukes 
among the parasites. 
The Tape - worm 
consists of the so- 
called head—the 
proper worm — and 
the body segments, 
I 
Fig. 216—Tape-worm (Taenia solium ): a, head; &, e, Fig. 217_Planarian 
d, segments of the body. worm. 
