46 ilfr Hardy and Mr Mc Dougall, On the Structure [Jan. 30, 



them but merely hurried through. They thus differ profoundly 

 in function from the similar structures in the higher forms of the 

 Crustacea. 



Minute structure of the Gut of Daphnia. 



The walls of the gut consist of a single-layered epithelium 

 resting on a muscular membrane. 



The Mesenteron including the liver diverticula is lined by 

 columnar cells disposed quite regularly and with a well developed 

 hyaline border with vertical striation. When the gut is in a state 

 of average normal distension the cells are short columnar, being 

 only a little deeper than broad. When the gut is much con- 

 stricted the epithelium is thrown into longitudinal rounded ridges 

 separated by furrows. The cells of the ridges are then very tall 

 and thin, those of the furrows short and broad. The gut may 

 be so distended by food or perhaps artificially during manipulation 

 that the cells are broader than deep. 



The depth of the hyaline border varies in different regions of 

 the gut but bears a fairly constant proportion to the depth of the 

 cells in each region. There can generally be seen embedded in the 

 substance of the border more darkly staining rods. They are most 

 commonly wholly embedded in the border, reaching just to its free 

 surface ; and are generally separated by spaces about equal to or 

 slightly greater than their own thickness. The distance which 

 separates them from one another varies with the degree of dis- 

 tension of the gut and the consequent breadth of the cells. When 

 the cells are very broad the border with the rods sometimes appears 

 discontinuous, there being patches of striated border on each cell, 

 separated by intervals. 



Sometimes no rods or structure of any kind can be made out 

 in the border ; this may be due to complete retraction of the 

 rods ; but possibly to faulty preparation. 



The rods may be sometimes seen to be shorter than the depth 

 of the border and so not quite to reach to its surface. 



In other cases the rods may be seen to project freely beyond 

 the border to at least half their length. In such cases the pro- 

 jection is not due to retraction of the border but to projection of 

 the rods, for the border retains its normal depth while the rods 

 are longer proportionally to the height of the cell ; they may be 

 more than half the height of the cell when thus projected. 



When the rods project through increase in length, they do not 

 seem to be narrower, but rather thicker ; if this be so, there must 

 be a passage into them of substance from the cell body. 



The rods are not always parallel and not always perpendicular, 

 sometimes they lie at an angle to the surface of the cells. 



The rods again may appear to stand freely on the surface of 

 the cells without any embedding hyaline substance. 



