INVERTEBRATA. 347 



nephridium by long delicate tubes, composed of a kind of 

 cuticular substance and opening into the cavity of the 

 nephridiuin. In the interior of these tubes is a long 

 vibrating flagellum, which is attached on the one hand 

 to the protoplasm of the cell, on the other extends into 

 the cavity of the nephridium. These cells have been named 

 solenocytes or tube-cells (solen, a pipe or tube; kutos, a 

 cell). In some cases it has been found that ciliated funnels 

 formed on the walls of the coelom, usually in the angle 

 between body-wall and septum, grow out into ducts, and 

 when the worm becomes sexually mature open into the 

 cavity of the nephridium. Such funnels and ducts act as 

 generative ducts. 



12. Characters of Nephridia and Coelomic Ducts. By 

 the consideration of such facts as these we are led to the 

 conclusion that nephridia and generative ducts are not 

 modifications of one kind of organ, but organs of two 

 distinct types, whose essential and primary characteristics 

 are strongly contrasted. The characteristics of a primitive 

 nephridium are : 



1. Its tube is intracellular not a tube lined by an 



epithelium, but a tubule pierced through the 

 actual substance of a scries of single cells (drain- 

 pipe cells). 



2. It develops from without inwards, or centripetally 



from the epiblast. 



3. It has no internal opening, but whether its internal 



extremity is simple or branched, it ends blindly in 

 flame-cells, or cells provided with a vibratile 

 flagellum. 



The characteristics of a coelomic duct, which is primarily 

 a generative duct, on the other hand are : 



1. Its tube is intercellular : it is a tube lined by an 



epithelium. 



2. It develops in the mesoblast from the coelom out- 



wards. 



3. It has necessarily a coelomic opening, because this 



is the first part developed, and this opening is a 

 ciliated funnel. 



