DIGESTIVE TISSUES 



291 



orly over other columnar cells to be exposed on the radular surface 

 where they function as cutting and masticating structures. 



The honey-sac of a bee is the dilated posterior end of the stomadceum. 

 The wall of the sac has a muscular coat. It is lined with a low columnar 

 epithelium which secretes 

 a dense cuticle. At the 

 posterior end the sac is 

 thrown into a rounded 

 prominence which pro- 

 jects anteriorly into the 

 lumen of the sac. The 

 cuticle over the curved 

 surface is modified to 

 form numerous pointed 

 processes, which are di- 

 rected towards the open- 

 ing at the apex of the 

 prominence. This open- 

 ing leads through a narrow 

 passage into the intestine. 

 The wall of this passage is 

 highly muscular. Its lu- 

 men has many long, cu- 

 ticular setae which are 

 directed posteriorly (Fig. 

 254). These two sets of 

 cuticular processes func- 

 tion as mechanical aids to 

 digestion. By means of 

 them nectar or pollen is 

 passed into the intestine as demanded. When pollen is demanded, the 

 short processes, by their action, carry pollen grains with a certain 

 amount of nectar into the narrow passage leading to the intestine. As 

 this is being done the passage is closed posteriorly. A constriction 

 then passes anteriorly, according to Cheshire, sending the nectar back 

 into the honey-sac and leaving the pollen grains held by the long, 

 cuticular setae. These then pass the pollen into the intestine with 

 little or no nectar. 



Teeth. The structure of a tooth is perhaps best studied in a series 

 of developing teeth. We have chosen the teeth of a dogfish for this 

 study. In the embryo dogfish the mouth is lined with a stratified 

 epithelium, the basal cells of which tend to be columnar. Along the 

 inner margin of the jaw this stratified epithelium forms a crescent-shaped 



FIG. 254. Longitudinal section of honey-sac of bee. 

 />., pointed processes on curved epithelial surface; 

 5., setae on inner lining of muscular passage. (From 

 PACKARD after CHESHIRE.) 



