THE Ca<]LOM AND NBPHRIDIA OF PALiEMON SERRATUS. 165 



already been seen, by means of a leather long, narrow tube, wliich is 

 beset witli small c^ca, and wliich from about its middle point gives 

 off a long branched tube, which ramifies about among the tissues of 

 the base of the eye-stalks and of the first antennae., Similar tubules 

 are given off from the bladder, running from this organ into the 

 second antenna. All these caecal appendages of the coelomic system 

 are lined throughout by an epithelium, which is perfectly character- 

 istic, and by which they are easily to be recognised in sections of 

 the eye-stalks or antennae. 



In a paper read at the meeting of the British Association at Man- 

 chester, in 1887, an abstract of which was subsequently published 

 in Nature (vol. xxxvii. No. 960, March 22nd, 1888, p. 498), Lan- 

 kester has described certain spaces in the limbs of Astacus which 

 appear to be lined by an epithelium and to be distinct from the 

 blood-vascular system of spaces. It seems possible that these 

 spaces, which Professor Lankester considers to be ccelomic in nature, 

 are derived from processes of the nephridio- coelomic apparatus of 

 the same nature as those just described in Paloemon. 



The external appearance of the nephridium itself has been accu- 

 rately described by Grrobben, except for the omission of the coelomic 

 canal which enters the bladder at the postero-internal angle, and 

 which was entirely overlooked by this author, whose mistake in this 

 point is probably due to his method of dissection, for he begins his 

 account of the kidney with the words, " Praparirt man die (hintere) 

 Antenne los," in which case one could certainly not expect a com- 

 munication between the nephridium and any structure in the trunk 

 to remain unbroken. 



The nephridium communicates with the exterior by a short, deli- 

 cate ureter, opening in the ordinary position at the base of the first 

 antenna, and which opens by its proximal extremity into the antero- 

 internal angle of the bladder. 



The bladder itself is large, and its outer wall is invaginated by 

 the glandular portion of the kidney and by the "end-sac;" these 

 last-named structures being therefore partially invested by the epi- 

 thelium of the bladder, in the manner described by Grobben in 

 P. Treillianus. The layer of epithelium thus investing a part of 

 the kidney is shown in fig. 8. 



The epithelium of the bladder varies in character in various 

 regions. That portion of the wall which forms the investment of 

 the end-sac and of the glandular tubules consists of a layer of 

 flattened cells, the nuclei of which stain deeply with hgematoxylin, 

 and appear nearly homogeneous, the protoplasm of the cells being 

 granular, and also staining deeply (fig. 7). In the free portions. 



