132 THE COCKROACH. 



Dytiscus). The crop is filled with food coarsely divided by the 

 mandibles, and the gizzard being shut to prevent further 

 passage, the fluid secretion of the cceca ascends to the crop, and 

 there acts upon the food. Digestion is effected in the crop, and 

 not beyond it. This is clear beyond doubt. In Decapod Crus- 

 tacea also it is very easy to prove that the fluid secreted by the 

 so-called liver ascends into the stomach (which corresponds to 

 the crop, together with the gizzard of the Insect). To satisfy 

 ourselves on this point we have only to open a Crayfish during 

 active digestion. 



When digestion in the crop is finished, the gizzard relaxes, 

 and the contents of the crop, now in a semi-fluid condition, 

 pass into the mesenteron, which is devoid of chitinous lining, 

 and particularly fitted for absorption. 



5. --There are no absorbent vessels properly so called, and 

 Plateau has long thought that the products of digestion pass by 

 osmosis directly through the walls of the digestive tube, to mix 

 with the blood in the perivisceral space. If we may rely upon 

 what is now known of the process in Vertebrates, we should be 

 led to modify this explanation. It is very likely that in Insects, 

 as in Vertebrates, absorption is effected by the protoplasm of 

 the epithelial cells, which select and appropriate certain sub- 

 tances formed out of the dissolved food. Not only do the 

 epithelial cells transmit to the neighbouring blood-currents the 

 materials which they have previously absorbed, but they subject 

 certain kinds to further elaboration. The protoplasm of the 

 epithelial cells of Vertebrates is capable of forming fat. Tims, 

 a mixture of soap and glycerine, injected into the intestine of 

 a Vertebrate, is absorbed by the lacteals in the form of oil- 

 drops. Modern physiologists allow, too, that part of the 

 peptone is similarly changed into albumen, without transport to 

 a distance, by the activity of the epithelial lining. 



These facts explain why Plateau was unable to isolate the 

 secretion of the epithelium of the chylific stomach of Insects. 

 The cells are not secretory, but absorbent ; and the secretion 

 vainly sought for does not actually exist. 



