DIGESTION EST MTJLTICBLLTJLAE ANTMAT.R 151 



ticular interest in showing how the needs of the animal 

 have brought about adaptations of a straight tube to 

 digestive purposes. For example, the widening at the 

 pharynx is a modification to suit the earth burrowing 

 habits of the animal and permit the taking in of food of 

 a solid character. The saclike enlargement back of the 

 pharynx, called the crop, permits the food to be stored 

 for a time in the tract, and enables the animal to eat 

 more at one time than he can digest, and thus give the 

 parts a chance for rest. The thick walled portion back 

 of the crop, called the gizzard, is merely a portion whose 

 muscle layers are thickened to form a special grinding 

 organ for macerating the food, since the animal possesses 

 no teeth. Furthermore, all these modifications are made 

 in a tube of essentially the same general structure as our 

 own tracts, since it has an inner layer of secreting cells, 

 a middle muscle layer, and just outside it is a blood fluid 

 to receive the absorbed food, as in our own body. In 

 fact we might almost say that the earthworm's digestive 

 tract is a living model of what a simple human tract 

 might have been before it was modified into stomach 

 glands, etc. 



In the earthworm, therefore, we have a distinct advance 

 over the hydra and sponge in that certain cells have been 

 set aside to make a tube in which to digest the food for 

 the whole body. Further, the manner in which this tube 

 is constructed and modified indicates the way in which 

 the complex system of glands, cavities, and organs of the 

 higher animals could have been developed from such a 

 simple foundation to fit the needs of the animal. As we 

 pass from the earthworm to the higher animals the in- 

 creased amounts of food necessary to furnish a much 



