446 EDWIN S. GOODRICH. 



the ciliated organ varies considerably in development in 

 different individuals. Not always does it reach the body- wall, 

 and I am inclined to think, after examining a very large 

 number of worms, that it is more developed in those animals 

 which approach sexual maturity. 



In the small species, Gl. convolutus, the ciliated organ is 

 relatively little developed, without the pointed flaps on either 

 side of the entrance into the nephridial sac, and never reaching 

 the body-wall so far as I have been able to make out (figs. 13 

 and 21J. 



The Nephridial Sac. — As already mentioned, this in- 

 teresting organ consists of a round flattened sac, over which 

 the nephridium lies. It must be clearly understood that 

 structurally, and no doubt also in its origin, it is quite 

 distinct from the nephridium, and is only called 

 "nephridial" because of its close connection with that organ. 



It is, in fact, a hollow sac formed by a layer of flattened 

 epithelium continuous with the epithelium of the ciliated 

 organ, and communicating only with the ccelom by means of a 

 single opening (figs. 2, 3, 16, and 30). 



Just as the ciliated organ is to be considered as formed of 

 differentiated coelomic epithelium, so no doubt the sac is 

 essentially a mere pouching of this same epithelium covering 

 the nephridium at that region towards which the ciliary action 

 of the ciliated organ converges — or, in other words, at the point 

 where this organ accumulates the cells floating in the coelom. 



In Gl. convolutus the nephridial sac is quite a small 

 chamber (figs. 13 and 21), into which the lip of the ciliated 

 organ is produced, forming its wall on one side. 



In connection with the quite anterior nephridia of Gl. 

 unicornis and Gl. siphonostoma the sac is not much more 

 developed than in the first species (fig. 11); but farther back 

 it gets larger and larger, until it becomes a relatively huge and 

 almost spherical structure (fig. 3). 



Moreover these large nephridial sacs become differentiated 

 into two regions — a main rounded sac which opens to the 

 coelom, and a diverticulum from the main sac, to the blind end 



