182 
THE WORM GROUP 
are able to tell the difference between night and day and to 
withdraw into their burrows if one steps heavily on the 
ground near them or takes a small stick and strikes the 
ground within a few feet of them. 
The organs of the earthworm are supplied with blood 
which is carried in a large dorsal blood vessel, a ventral 
blood vessel, and numerous branches. The blood is pumped 
by the contracting of the dorsal vessel 
and by the five pairs of tubes which pass 
from the dorsal to the ventral vessel 
around the esophagus. These five tubes 
are named aortic (a-or'tik) arches. 
LABORATORY STUDY OF INTERNAL 
STRUCTURE 
Work out the internal structure of the earth¬ 
worm. In dissecting, cut the skin along the 
dorsal surface, being careful to cut the many 
membranes that hold the digestive tube in place. 
Work out the size and position of the mouth 
cavity, pharynx, esophagus, crop, gizzard, and 
stomach-intestine. The white reproductive or¬ 
gans are located beside the esophagus. Locate 
the “brain,” the ventral chain of ganglia. The dorsal blood vessels and 
aortic arches should be located. Make a sketch locating the organs in 
their respective segments. 
167. Life History. — In the starfish group the sexes are 
distinct. The sexes in the annelids are distinct in some forms 
and in others the same individuals have both ovaries and 
spermaries. The sperms, however, that unite with eggs 
always come from another worm. During the season when 
the ovaries and spermaries are forming eggs and sperms, 
certain segments, usually six in number, beginning with the 
twenty-eighth segment, and known as the clitellum (kli- 
tel'lum), pour out a gelatinous secretion which hardens into 
a collar-like sac around the worm. 
Figure 168. —Front 
End of the Nerv¬ 
ous System of the 
Earthworm. 
The large mass is 
termed the “brain” 
because of its size 
and position. 
