280 W. F. R. WELDON. 



a short summary of the results already arrived at concerning 

 the structure of the excretory apparatus in the Prawn. 



The nephro-peritoneal sac of Paisemon is a median, 

 unpaired structure, lying in the cephalothorax, dorsal to the 

 alimentary canal, and ventral to the ophthalmic artery and to 

 the median dorsal blood-sinus. Its walls are composed of a 

 single layer of flattened excretory epithelium, which has the 

 power of absorbing indigo carmine and similar substances 

 when these are injected into the blood of the living animal. 

 Posteriorly the nephro-peritoneal sac is in close contact with 

 the anterior extremity of the generative gland ; anteriorly it 

 gives off on each side a narrow tube, which passes vertically 

 downwards beside the oesophagus, and, after passing under the 

 oesophageal nerve-commissure, bends outwards to open into 

 the urinary bladder of its own side. The ducts of opposite 

 sides communicate with one another, not only dorsally, through 

 the cavity of the nephro-peritoneal sac, but also vertically, by 

 means of a transverse commissure which passes in front of the 

 oesophagus, and bears a conspicuous dilatation in the cavity of 

 the upper lip. 



A diagram of the whole arrangement is given in PL XXII, 

 fig. 9 ; the relations to the other systems of organs, as seen in 

 transverse section, may be gathered from figs. 1 and 2. 



The bladder, besides receiving the openings of the nephro- 

 peritoneal ducts, gives off on the one hand the ureter, and on 

 the other the system of excretory tubules of the green gland. 

 These last form a complex mass of branching tubules, which 

 are in close contact one with another, and which form the 

 glandular substance of the green gland. In my former paper 

 I pointed out the existence, in this glandular plexus, of several 

 excretory tubules, an observation confirmed by M. Marchal. 

 I was unable last year, and have still been unable, to convince 

 myself that the various tubules anastomose freely with one 

 another in the way described by this observer. Be this as it 

 may, however, the tubules, after a tortuous passage through 

 the substance of the green gland, unite to open by a common 

 aperture into the " end-sac," whose structure has been re- 



