FUNCTION AND STRUCTURE 



71 



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 \&L G, 



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 oesophagus. Behind the oesopha- 

 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- FIG. 34.-Earthworm diBsected 



. -i T-I J.T to show alimentary canal, 



ber called the gizzard. From the ^ c 



gizzard back the alimentary canal is 



about uniform in size, being rather wide and having thick, 



soft walls. This portion of it is called the intestine. The 



