288 W. F. R. WELDON. 



fig. 8) exhibits already the characteristic longitudinal striation, 

 and as in the young Pandalus it surrounds half the end-sac. 

 The communication between its cavity and that of the end-sac 

 is direct, there being no trace of the renal tubule. 



The end-sac is bounded by a layer of pale, finely granular 

 cells, the protoplasm of which exhibits the well-known ^^ground- 

 glass " appearance. The epithelium of that half of the sac 

 which is enclosed by the bladder is more columnar, that of 

 the unenclosed portion being flatter. The cells of both regions 

 exhibit numerous vacuoles at their inner margins, but no con- 

 cretions were observed. 



Between the end-sac and the bladder is a well-developed, 

 apparently lacunar blood-space, and outside the end-sac is a 

 layer of connective tissue. 



The excretory system of an adult shrimp resembles that of 

 Pandalus in the direct communication between end-sac and 

 bladder, and in the formation of papillae upon that surface of 

 the end-sac which projects into the bladder. The whole sac is 

 not, however, enveloped by the bladder so completely as it is 

 in Pandalus. 



It is evident from what has been said that the excretory 

 system of the Decapoda is much more varied in its structure 

 than has hitherto been supposed. The observations here 

 recorded, together with those of M. Marchal already referred 

 to, enable us to divide the modifications into groups as follows : 



In the Schizopods (Mysis) the whole excretory system 

 appears, according to Grobben,^ to consist of a single coiled 

 tubule, opening by one extremity to the exterior, and by the 

 other to an irregular end-sac, whose walls are composed of an 

 irregular epithelium, and are not apparently very highly 

 specialised. The single renal tubule may dilate into a small 

 bladder near its external opening, but there is no indication of 

 the extension into the thorax of a nephro-peritoneal sac. 



In all the Decapods proper the end-sac has become more 

 highly specialised, possessing a lining epithelium of definite 

 characters, a well-defined system of blood-vessels, and so on ; 



' ' Arb. Zool. Inat. Wien,' iii, 1881. 



