DIGESTIVE TISSUES 



299 



propria, or basement membrane. The wall of the tubule is relatively 

 thick and incloses a narrow lumen. In this case the cells are prolifer- 

 ated at the fundus of the , ^ c> 



tubules and at certain \ 



i ^1-1 6. w. * 



regions along the sides. \ 



When immature they are 

 small with compact, dense 

 protoplasm. The nuclei 

 are oval to slightly irregu- 

 lar in shape. The cells 

 and their nuclei increase 

 in size. The cytoplasm 

 becomes vacuolated at the 

 distal ends. Within these 

 vacuoles secretion, and 



FIG. 268. Cells from the digestive gland of Mesodon 

 (Helix), cal.ph.c., calcium phosphate cell. Others are 

 hepato- pancreatic cells, b.m., basement membrane, on 

 which lies a narrow connective nucleus. X 970. 



-i oZ. c. 



perhaps certain excretion, 



products appear (Fig. 267, A and B). When the cell is fully grown it has 

 attained a great size, and bears an immense vacuole which crowds most 

 of the cytoplasm and the nucleus to the base or one side of the cell (Fig. 

 267, A). 



The general digestive epithelium of mollusks is strongly ciliated 

 (see Fig. 52). In the connection with this ciliated tissue gastric glands 



have been developed. As an example of 

 this more highly specialized tissue we shall 

 take the so-called hepato-pancreatic gland 

 of Mesodon. The chief cells of this greatly 

 branched gland are columnar cells, as 

 shown in Figure 268. These cells secrete 

 a ferment that aids in digestion. They 

 have also the power to elaborate glycogen. 

 In addition to the chief cells an occa- 

 sional albumen cell and as frequently a 

 calcium phosphate cell is met with. 



The digestive tissues of Amphioxus are 



Fte. 269, 4 and *.-^ digestive epi- but ^ little Specialized. The intestinal epi- 



thelium from intestine of Amphi- thelium is carried into the hepatic ccecum 



OXUS . al.c albumen cells which Qr ^ , d This epithelium is com _ 

 bear no cilia. X 1250. B, enlarged ' . r 



ends of the same sort of epithelium posed of very slender ciliated cells. Each 

 to show relations of cilia, 



(B is after SCHNEIDER.) 



x 3000. cdl accon }i n g to Schneider's figure bears 



' 



a single cilmm. Occasional albumen cells 

 are found lying among the ciliated digestive cells. The nuclei of these 

 cells lie farther from the basement membrane than do the nuclei of 

 the ciliated digestive cells (Fig. 269). 



