6. THE COELOM AND ITS DERIVATIVES - THE EXCRETORY SYSTEM 69 



am 



cex 



Fig. 47. Left coelomoduct of the protosoma of 

 LameUisabella zachsi, seen from the left side: a 

 graphic reconstruction from sections. 

 ad - anastomosing canal of coelomoducts ; am - 

 ampulla ; cex - distal canal of coelomoduct ; 

 cin - proximal canal of coelomoduct ; ex - excre- 

 tory portion ofcoelomoduct;^ - external pore of 

 coelomoduct; sn - "renal sac" - an invagination 

 of the ventral wall of the dorsal vessel in which lie 

 the excretory portions of the coelomoducts. 



the walls of these canals consist of cubical ciliated cells. The excretory section 

 is formed of rounded nephrocytes, furnished with bundles of long cilia and 

 stuffed with brown excretory granules, which are not infrequently encoun- 

 tered in the lumen of the organ also (Fig. 48 Д B). The excretory duct 

 consists of low ciliated cells. Finally the ampullae lie wholly within the thick- 

 ness of the epidermis and to some extent look like mere invaginations, but in 

 Spirobrachia and LameUisabella they consist of two parts — a proximal ex- 

 pansion, formed of ciliated cells, and a distal external part with a dilated 

 lumen, clad with a thin layer of cuticle instead of cilia (Fig. 48C). 



Concentrations of granules in the cells of the coleomoducts reveal by their 

 brown colour that they are excretory inclusions. They increase greatly in 

 number towards the distal end of the organ, particularly in the larger 

 Thecanephria, and in the regions in close contact with the blood vessels they 

 account for the intensely staining nature of the coelomoducts. Excretory 

 matter probably passes into them from the blood, and to all appearances this 

 is their chief function. The coelomoducts may also serve to regulate the 

 amount of body fluid in the protocoele and hence the amount in the coelomic 

 canals of the tentacles also. 



