40 



FIRST LES&OlTB IN ZOOLOGY 



typical worm is bilateral, with a well marked dorsal and ventral »ide and 

 a head-eiid arid tail-end, with tlie body divided into segments. 



By dissecting the body and tracing with needles the internal anato- 

 my, and also by cross-sections of the body, tlie following relation of 

 the most important internal organs will be observed. The digestive 

 tract is a slender tube lying free in the body-cavity, and extend- 

 ing from the mouth to the vent. Above it lies a long delicate pul- 

 sating 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. 37). one for nearly each seg- 

 ment. A worm may or may not have eyes. The flat- worms have 

 two (Fig. 38), or many scattered all over the body; others have eyes 

 both in the head and tail; many worms have ears — i.e , organs of 

 hearing. All worms grow from eggs, and the sea-worms have free- 

 swimming young entirely unlike the parent worm, which pass through 

 a metamorphosis. 



The common earth-worm is cylindrical and many-jointed. 

 The small mouth opens on the under side of the first seg- 



FiG. 38. — Planaria torva (en- 

 larged); and e, egg-capsule. Fig. i^.—Macrnbdella, or pond-leech. 

 (Natural size.) (Natural size.) 



ment. The earth-worm is able to climb perpendicularly 

 up boards or the sides of buildings by minute; short, curved 

 bristles, which are deeply inserted in the muscular walls of 

 the body, and arranged in two double rows along each side 



