FUNCTION AND STRUCTURE val 
connects the mouth with the rest of 
the alimentary canal, 
which gives out many side branches or diverticula, which 
are themselves branched, so that the 
alimentary sac or stomach is a system 
of ramifying tubes extending from a 
central main tube to all parts of the 
body of the worm. There is no 
anal opening. In the round or thread 
worms, of which the deadly Trichina 
is an example, the alimentary canal 
is a simple straight tube with both 
anterior or mouth opening and pos- 
terior or anal opening. In the sea- 
urchins and sea-cucumbers (Fig. 33) 
the alimentary canal is a simple tube 
with two openings, but it is longer 
than the body between mouth and 
anus, and so is more or less bent or 
coiled. In the earthworm the ali- 
mentary canal (Fig. 34), although a 
simple straight tube running through 
the body, plainly shows a differentia- 
tion into particular regions. Behind 
the mouth opening the alimentary 
tube is large and thick -walled and 
is called the pharynx; behind the 
pharynx it is narrower and is called 
the esophagus. Behind the csopha- 
gus it expands to form a rounded, 
thin-walled chamber called the crop, 
and just behind this there is another 
rounded but very thick-walled cham- 
ber called the gizzard. From the 
gizzard back the alimentary canal is 
Fig. 34.—Earthworm dissected 
to show alimentary canal, 
al. ¢. 
about uniform in size, being rather wide and having thick, 
soft walls. This portion of it is called the intestine. The 
