284 HISTOLOGY 



The serous tissues of digestion elaborate a watery secretion that con- 

 tains the ferment ptyalin or some other digestive ferment. In secreting 

 ptyalin they resemble pancreatic tissues. The serous tissues functionally 

 represent grouped albumen cells or serocytes, which are so frequently 

 encountered isolated among 'the alimentary tissues of invertebrates. 

 The parotid gland and certain lingual glands at the base of the tongue 

 are pure serous glands. The submaxillary gland presents both mucous 

 and serous cells ; hence it is called a mixed gland. 



The hepatic tissues secrete bile, which is a fluid active in the digestion 

 of fats. This tissue also has the power to elaborate glycogen from cer- 

 tain soluble carbohydrates. With this function the hepatic tissue be- 

 comes a storehouse of energy. In the lower forms other tissues than 

 the gland known as the liver may have this accessory function, as the 

 so-called liver cells found in the foot and dorsal mantle region of the 



tr. 



f. m. 



FIG. 245. Individual of Paramcecium caudatum. Arrows show course of food vacuoles (f.v.). 

 nu., nuclei; con.v., contracting vacuoles, one empty and one full; f.m., fecal matter; tr., 

 discharged trichocyst. X 375. 



fresh-water mussel. All vertebrates have hepatic glands that elaborate 

 glycogen. 



Certain alimentary tissues have been differentiated as structures no 

 longer directly concerned with alimentation, as for example the poison 

 glands of certain reptiles. These will be considered under another 

 heading. 



Examples of intra-cellular alimentary structures. Paramcecium is 

 a very common protozoon that is found in most infusions. It is a 

 slipper-shaped creature with a rounded, narrow anterior extremity 

 and pointed at the posterior end. Extending from the anterior to the 

 middle of the body there is a lateral oral groove, which leads in a slight 

 spiral manner to the gullet at its posterior end. By means of cilia 

 currents of water are created, which bear food along the oral groove 

 into the gullet. At the base of the gullet the food and water taken in 

 with it form a spherical food vacuole. The vacuole becomes too 

 large to withstand the impact of the water entering it from the gullet 

 and breaks away. It is then slowly carried along with the cyclosis of 



