EENAL ORGANS OP CERTAIN DECAPOD CRUSTACEA. 285 



tubule and end-sac ; its cells are, for the most part, pale and 

 uniformly granular, having no trace of longitudinal striation ; 

 their nuclei are rounded, and do not stain deeply. At this 

 stage there is little diflPerence in appearance between that por- 

 tion of the bladder-wall which is immediately adjacent to the 

 end-sac and the remainder. The cells adjoining the end-sac 

 are slightly smaller than the rest, and their margins slightly 

 more regular, but in other ways the characters of the bladder- 

 cells are everywhere the same ; and the sac, though closely 

 applied to the wall of the bladder, causes hardly any invagina- 

 tion of that structure. 



The renal tubule (PI. XXI, fig. 5) is very short, being 

 simply represented by the curved neck which connects the 

 end-sac with the bladder. Its walls are very similar in struc- 

 ture to those of the bladder itself, and its lumen is very narrow. 



The end -sac (PI. XXI, fig. 5) is small, and bounded by 

 finely granular or nearly homogeneous cells, the nuclei of 

 which are clear and vesicular in appearance. Between the 

 end-sac and the wall of the bladder is a lacunar blood-space ; 

 and both here and on the outer side of the end-sac are groups 

 of connective-tissue cells. 



In a very young Pandalus annulicornis, which had 

 apparently only just acquired the adult characters, the appear- 

 ance presented by a transverse section through the excretory 

 organs is shown in fig. 6. [I have only obtained a single 

 individual of this age, and before the present investigation 

 was commenced I had prepared transverse sections of the 

 specimen. I have therefore been unable to figure a section 

 in the same plane as those above described.] 



The wall of the bladder exhibits already a distinct specialisa- 

 tion into two regions, one lying beside the end-sac, the other 

 having no relation with that organ. In the latter portion, 

 which of course includes the greater part of the bladder, the 

 cells exhibit distinct indications of longitudinal striation in 

 their peripheral parts, and their inner extremities are often 

 vacuolated. The portion which adjoins the end-sac consists of 

 cells which are more columnar than those of the general 



