170 



GENERAL PHYSIOLOGY 



cells, which in the compound organism play so very important a 

 Tuh in protecting the internal surfaces and keeping them smooth 

 and moist by their secretion of mucus, are always cylindrical. 

 The nucleus, surrounded by somewhat more solid proto23lasm, lies 

 at the bottom of the cell-body, while the upper end of the cell, 

 which borders the free surface of the membrane, is formed by a 



A B 



Fig. 62. — An AmahaiTi four succe.ssive stages of excretion of the undigested residue of food. 



substance, mucigen, that i.s continually being transformed into 

 mucus. During the quiet activity of the cell a little of the 

 secretion passes constantly to the thin liquid layer that covers the 

 surface of the tissue. But during energetic, sudden secretion 

 the whole mass that forms the upper part of the cell is shoved 

 out (Fig. 63) and blends with the drops cast out of the neigh- 

 bouring cells into a thick, gummy covering of mucus. The 



Fig. 03.— Mucous cells. A, Three isolated cells. B, Seven adjacent colls, of which the three at the 

 left are full, the four at the right are empty. (After Schieffei-decker.) 



peculiarity of many holothurians, those cucumber-shaped forms 

 of echinoderms, of transforming their thick, solid skins upon 

 stimulation in a short time into a glistening, viscous slime, is very 

 remarkable and not yet explained. In general, the cell-physio- 

 logical investigation of the process of secretion promises to afford 

 many very interesting general physiological facts. 



