142 



ANNELIDA 



[CH. 



of segments thirteen to sixteen, and this is separated by a groove 

 from a thick-walled sac, with hard, horny walls, termed the gizzard, 

 which extends to about the twentieth segment. The exact segment 

 in which the above-mentioned parts of the alimentary canal lie varies 

 with the amount of food they contain, the septa which are pierced 

 by them being stretched forward or backward according to their 

 state of fulness or emptiness. 



Behind the twentieth segment the intestine stretches without 

 change to the anus. It is a thin-walled tube, supported by the 



septa between each segment and 

 swelling out slightly in each 

 segment, so that it presents an 

 outline like a string of beads. 

 A deep fold, called the typhlo- 

 sole (Gr. rvcf>\6<;, blind; o-wX^V, 

 a gutter), runs along the upper 

 surface of the intestine, project- 

 ing into its cavity. Its presence 

 causes the wall of the intestine to 

 be pushed in, and thus its inter- 

 nal absorbing surface is increased 

 (7, Fig. 61). The intestine is 

 covered everywhere by a number 

 of cells of a yellow colour. 

 These form the inner wall of the 

 coelomic sac and are actively 

 engaged in excretion. 



The exact part that each of 

 the above-mentioned parts of 

 the alimentary canal plays in 

 digestion is not thoroughly 

 understood. The pharynx helps 

 to take food in by a sucking 

 action which is caused by the 

 contraction of the muscles run- 

 ning from it to the body-wall, 

 resulting in an enlargement of 

 the cavity of the pharynx so 

 that food may pass in by 

 atmospheric pressure. The food passes down the oesophagus, 

 being propelled by a series of contractions of the walls of the 



FIG. 60. Anterior view of the internal 

 organs of an Earthworm, Lumbricus 

 terrestris. Slightly magnified. From 

 Hatschek and Cori. 



1. Central ganglion or brain. 2. Mus- 

 cular pharynx. 3. Oesophagus. 

 4. Crop. 5. Muscular gizzard. 

 6. Intestine. 7. Nephridia (the 

 reference lines do not quite reach 

 the nephridia). 8. Septa. 



9. Dorsal blood-vessel. 10. Hearts. 

 11. Spermathecae. 12. Vesiculae 

 seminales. 



The Koman figures refer to the number 

 of the segments. 



