THE CRAYFISH 133 



and if its dorsal face is examined with a lens after removal of 

 the bladder, it may be seen to consist of the following parts : — 

 (i.) A triangular mass of brownish-yellow colour, occupying 

 the centre of the upper surface of the gland, the apex of the 

 triangle directed posteriorly : this is the end-sac. (2.) A 

 medullary mass of greyish-white colour surrounding the end- 

 sac. Closer inspection shows that this is composed of a 

 coiled tube, which opens by one end into the bladder by a 

 relatively large oval aperture, situated just behind the posterior 

 angle of the end-sac. The other end of the tube passes into 

 (3) the intermediate substance occupying the posterior part 

 of the gland, and this in turn is connected with (4) the 

 cortical substance, a layer of bright green tissue investing the 

 whole surface of the gland except the central and posterior 

 parts of its dorsal face. The end-sac may be described as a 

 vesicle whose cavity is divided into numerous chambers by 

 irregular partitions extending inwards from its walls. It com- 

 municates by several openings at its anterior end, with the 

 cortical substance, the latter being a labyrinth of reticular 

 canals lined by a cubical glandular epithelium containing 

 numerous green globules. The intermediate substance is 

 really a reflection of the cortical substance over the posterior 

 edge of the gland, and differs from it chiefly in the fact that 

 the green globules are not present in its epithelial cells. 

 The network of tubes forming the intermediate substance 

 opens by several apertures into a relatively wide transparent 

 tube, which is thrown into the several coils constituting the 

 medullary substance. The greater part of the cavity of this 

 tube is sub-divided by numerous irregular ingrowths from its 

 walls, so that it is of a spongy texture. Eventually the tube 

 opens just behind the posterior angle of the end-sac into the 

 bladder. The end-sac and all the regions of the labyrinth are 

 lined by a cubical epithelium, differing somewhat in character 

 in the different regions, but glandular throughout. It has 

 been shown that ammonium carminate injected into the blood- 

 vessels is taken up and excreted by the cells of the end-sac ; 

 indigo carmine similarly injected is taken up by the cells of the 

 labyrinth. Clearly, then, the different parts of the green gland 

 have different excretory activities, but as yet little is certainly 

 known about the composition of the fluid excreted. The 

 most that can be said, in view of recent contradictory state- 



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