DIGESTIVE TISSUES 



299 



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. 



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 , , . 



cal. ph- c - 

 tubules and at certain \ 



regions along the sides. * W? " 



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 



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,^). 



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 

 but little specialized. The intestinal epi- 

 thelium is carried into the hepaftic ccecum 

 or gastric gland. This epithelium is corn- 

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

 3 - cell according to Schneider's figure bears 

 a single cilium. 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). 



FIG. 269, A and B. A, digestive epi- 

 thelium from intestine of Amphi- 



to show relations of cilia, 



(B is after SCHNEIDER.) 



