38 



ZOOLOQT. 



i^ A 



Fio. 38 — " Brain" and part of 

 the nervous or ganglionated 

 cord of the earth-worm, h, 

 brain, or first pair of gang- 

 lia; g, nerves to pliarynx; 

 d, ixsophageal collar; /, 

 space occupied by the pha- 

 rynx ; 5-8, the ganglia of the 

 .5fh to 8th segments respec- 

 tively; !, nerves to first seg- 

 ment; nerve-threads orcom- 

 missures. From Brooks, af- 

 ter Lankester. 



Fig. Z^.—Plaiiaria iorva, en- 

 larged; ami e, egg capsule, 

 natural size. Gissler del. 



alike, the observer will soon be able 

 to distinguish tlie 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 ttlso by cross-sections of the 

 body, the following relations of the 

 most important internal organs will 

 be observed. The digestive tract 

 (i) 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 coru with swellings 

 called ganglia (Fig. 38), 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 



