48 
ZOOLOGY. 
— a. 
c. — 
— 
Fig. 50.—" Brain" and part of 
the nervous or ganglionated 
cord of the earth-worm, /i, 
brain, or first pair of gang- 
lia; gr, nerves to pharynx; 
oesophageal collar; /, 
space occupied by the pha- 
rynx; 5-8, the ganglia of the 
5th to 8th segments respec- 
tively ; 1, nerves to first seg- 
ment ; nerve-threads or com- 
missures. From Brooks, af- 
ter Lankester. 
Fig. 51. — Planaria torva, en- 
larged; and e, egg capsule, 
natural size. Grissler del. 
alike, the observer will soon be able 
to distinguish the head-end from the 
tail-end; he will also notice that both 
sides of the body repeat each other, 
and that there is an upper (dorsal) 
and lower (ventral) side, the worm 
lying on the latter side. The stu- 
dent will now be able to understand 
the following short definition of the 
branch of worms, to which there 
are some exceptions, which need not, 
however, be here mentioned. A 
typical worm is bilateral, with a well- 
marked dorsal and ventral side and 
a head-end and tail-end, with the 
body divided into segments. 
By dissecting the body and tracing 
with needles the internal anatomy, 
and ^Iso by cross-sections of the 
body, the following relations of the 
most important internal organs will 
be observed. The digestive tract 
(^) is a slender tube lying free in 
the body-cavity, and extending from 
the mouth to the vent. Above it 
lies a long delicate, pulsating tube 
called the dorsal vessel or heart. 
The brain is small, and is situated 
in the upper part of the head, while 
behind the throat on the floor of the 
body lies the main nervous system, 
a double white cord with swellings 
called ganglia (Fig. 50), one for 
nearly each segment. A worm may 
or may not have eyes: some kinds of 
worms have them scattered all over 
the body; others have eyes both in 
