DIGESTION 



197 



and swallowed which are much too large to be taken up by a 



cell. In the sea anemone such objects undergo partial dis- 



integration in the gastro-vascular cavity, and the fragments 



are taken up by the cells. The hydra lacks 



the organs by which this is accomplished by 



the anemone, but it is still probable that the 



close application of the walls of hydra to the 



prey may accomplish the same end. The 



undigested portions of the food are cast out 



at the mouth. 



455. The elongated form of the body of 

 the worm makes possible a considerable ad- 

 vance in the digestive system. The digestive 

 cavity is a slender tube opening to the ex- 

 terior at each end. The food is taken in at 

 the mouth, and as it passes slowly along the 

 narrow channel it is gradually digested and 

 absorbed. The parts that remain undigested 

 are cast out at the vent. The elongated 

 form makes possible the successive applica- 

 tion of different agencies of digestion to a 

 given particle and the simultaneous opera- 



FIG. 104. The in- 



tion of these agencies in different parts of testine of a worm 



the canal. In nereis there are jaws and 



denticles by which the food is captured and ing of the glandular 



forced into the mouth, and perhaps, to some body-wall' "is*' repre- 



extent, lacerated. There is a pair of "sail- ^ ed i 5 outl if e ' 



B.C., Body cavity; 



vary" glands which open into the anterior C.E., glandular epi- 

 end of the digestive tract and throughout ^ um ' } Int} l 

 the remainder of its length the intestinal epi- 

 thelium is thickly studded with unicellular glands, which also 

 pour a secretion into the digestive cavity. In the earthworm 

 the digestive canal is more differentiated. 

 456. In Worms we have unquestionably a case of a true 



