82 THE COCKROACH [CH. Ill 



the gizzard. This secretion emulsifies fats and oils, renders 

 proteids soluble, and completes the conversion of starch 

 into sugar. Thus all ordinary food substances are made 

 capable of diffusing through the walls of the crop and of 

 the chylific stomach into the blood. Examination of the 

 faeces gives reason for thinking that at least some cellulose 

 undergoes digestion, but I am not aware of any exact ob- 

 servations on this point. In the gizzard a certain subsidiary 

 amount of grinding is effected by the six hard longitudinal 

 ridges projecting from the inner face, but the chief use of 

 this organ is to act as a strainer and allow only very fine 

 particles to penetrate the sieve formed by its fine inter- 

 lacing hairs. By the time the food reaches the rectum, 

 all digestible substances have been rendered soluble and 

 absorbed into the blood ; the faecal residue is moulded into 

 pellets in this last section and expelled per anum. The 

 six longitudinal ridges which project into the rectal cavity 

 probably assist the expulsion by giving the walls a firmer 

 grip upon the contents and also give the slight screwing 

 movement observable in the faeces as they are forced out. 

 Similar screw-like twists are to be seen as the solid and 

 liquid ejecta of many other animals leave the body. The 

 muscular effort required to effect any such expulsion is 

 appreciably lessened in intensity by calling into use the 

 mechanical principle of the screw. 



Excretion. It is convenient here to deal with the system 

 for excretion of nitrogenous waste material inasmuch as 

 the Malpighian tubules, which are the organs concerned, 

 are attached to and open into the posterior end of the 

 chylific stomach near its junction with the intestine. 



