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- FIG 254 . _ Longitudinal section of honey , sac of ^ 



tlOn as mechanical aids tO p., pointed processes on curved epithelial surface; 

 digestion. By means Of , set* on inner lining of muscular passage. (From 

 3 PACKARD after CHESHIRE.) 



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 se.tae. 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 



