180 
THE WORM GROUP 
LABORATORY STUDY 
One of the annelids should be studied with some care, as an illustra¬ 
tion of an invertebrate animal. How do you determine the anterior and 
posterior ends? Dorsal and ventral surfaces? The number of seg¬ 
ments? Compare several worms. The back region of the worm shows 
the most variation because new segments are being added. Where are 
the setae ? How does the earthworm move ? Place it on a glass. The 
front region of the body is most sensitive to touch. Test it. 
163. Food-taking. •— The food of the earthworm is chiefly 
the soil in which it burrows. By means of an upper lip, 
which is a specialized anterior segment, and the muscular 
walls of the pharynx it takes the earth into its body, and 
the muscles of the digestive tube advance the food along 
its course. The soluble and therefore digestible parts 
are absorbed, and the remainder (the greater portion) 
is passed along to the outside. Earthworms are not critical 
in the selection of their food, although they are not entirely 
without a sense of taste. 
164. Respiration. — Oxygen passes through the skin 
directly into the blood vessels. The blood carries the oxygen 
to all parts of the body. Carbon dioxide passes from the 
blood through the skin to the outside. This interchange of 
oxygen and carbon dioxide is brought about by osmosis. 
165. Excretion. — In each segment is found a pair of 
organs known as nephridia which look like little threads. 
These organs remove the liquid waste and carry it to the 
outside of the body. 
166. Internal Structure of Earthworms. — This is shown 
diagrammatically in Figure 167. The internal structure 
consists of an outer tube, the body-wall, and an inner tube, 
the digestive tube. The space between the body-wall and 
digestive tube is known as the body cavity or ccelome (se'lum : 
Greek, koilos, hollow). Thin sheets of membrane pass from 
each furrow between the segments to the digestive tube. 
Beginning at the front end the digestive tube is given 
