338 TKAXSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



in which occur scattered muscle fibres (fig. 52). The 

 blood spaces are extremely narrow but can be traced 

 very easily after staining with methyl-blue-eosin (Mann). 

 Everj' now and then one meets with sections of the vessels 

 which occur on the ridges (see vascular system for further 

 details of circulation in renal organ.) The ridges con- 

 sidered above belong to what has been termed by Perrier 

 the " pi'inciiyal system." 



Running round the left side of the renal organ 

 proper, between it and the nephridial gland, is a mem- 

 branous flap ending in a smooth transparent edge (figs. 

 49 and 51, ren. lam.). It is continued, though very 

 much smaller in size, along the posterior margin. From 

 this lamella a number of secondary lamellae arise and run 

 at right angles attached to the wall of the renal organ 

 between the ridges already disciLssed. These little 

 partitions (fig. 51, lam. pr.) are hidden by the ridges, 

 but can be seen quite distinctly if the latter are just 

 pulled aside. In order to examine the structures further 

 it will be necessary to macerate the gland — and to remove 

 the glandular filaments entirely. 



It will tlien be seen that the secondary lamellae 

 give off in their turn tertiary lateral lamellae, 

 which finally divide up into very delicate branches 

 (fig. 51. htm. S.). This secondary system does not 

 appear to play any great part in the role of 

 excretion. It seems to be confined to the highest 

 prosobranclis, ;ind Perrier regards the two systems as 

 equivalent to Die two lobes of the renal organ of the 

 VolutidiK'. Tlic histological structure of these lamellae 

 is very different from that of the filaments. In section 

 they are of considerable tliickiiess. There is the same 

 bounding layer of glandular cells, but they are more 

 cubical than columnar. The greater part of the structure 



