CHAPTER V. 
FIFTH BRANCH OF ANIMALS. 
THE WORMS (Vermes). 
General Characteristics —Animals that as a rule have a 
head, tail, upper and lower surfaces (dorsal and ventral), 
and generally made up of an indefinite number of joints 
or segments. An idea of the internal structure can be 
obtained from the section of a leech (Fig. 48). The di- 
gestive organ is tubular, extending from the mouth to the 
vent. The heart is a small pulsating organ above it, and a 
system of vessels containing a clear red or green fluid lead 
into the respiratory organs when present ; many worms, 
however, breathe through the body-wall. The nervous 
system consists of a white double chain of ganglia extend- 
ing along the ventral surface. The brain is small, and 
situated in the upper part of the head. In some, the eyes 
are in different parts of the body—the head, tail, etc. ; in 
others, they are absent. In the leech, they are confined to 
the first three segments. In some, as the worms of the 
first class, two branching tubes are found that constitute a 
water vascular system similar to that of the Echinoderms. 
All the worms are developed from eggs, and many pass 
through remarkable changes before assuming the adult 
form. The worms include many animals that, though 
varying greatly in appearance, all possess certain charac- 
teristics in common. 
