1902.] 



NATURAL SCIENCES OF PHILADELPHIA. 



339 



undergone is due to the collection of muscle fibres into a more or 

 less distinct band which takes a spiral course. INIauifestly the peris- 

 taltic wave, beginning at the distal end and proceeding with greatest 

 vigor along this band, will be more effective for producing a uni- 

 form and constant motion of the fluid contents than would the 

 successive contractions of many separate bands. 



The epithelium in the average physiological condition is com- 

 posed, morphologically speaking, of two kinds of cells, namely, tall 

 conical cells which project well into the lumen (often, indeed, 



Fig. '21. — Section of single tube of the liepatopancreas of Porcellio 

 scaher X 120, showing discharge of secretion bj'^ fragmentation of the 

 cells. The cell a is represented with higher magnification in fig. 22. 



almost meeting each other so as to obliterate the lumen) and, 

 between these, lower cells which project less — often not at all (fig. 

 20). At both extreme ends of the tube the two forms of cells 

 merge into one another and thereby constitute a uniform epithelium 

 which, at the distal end, terminates in a proliferating mass of 

 indifferent cells, and, at the proximal, passes over into the 

 epithelium lining the grinding stomach. The cells of the hepato- 

 pancreas reveal a ground structure in .some points like that of the 



