i] STRUCTURAL AND SYSTEMATIC 37 



are placed round the vent, and are ciliated. Other 

 genera are Nais, Chaetogaster, Vejdovshyella, Am- 

 phichaeta, Stylaria, Macroehaetina, PHstina, 

 Naidium. 



Several genera, Pristina, Nais, Dero, are found 

 in many parts of the world; but it is not possible 

 at present to consider very seriously the facts of their 

 geographical distribution. 



Next to the Naids a group of aquatic worms 

 present themselves for consideration which are 

 usually placed in three distinct families, which 

 families are however rather hard to define. These 

 three families are the Tubificidae, Phi'eodrilidae, 

 and Lumbriculidae. The Phreodrilidae were at one 

 time placed with the Tubificidae by Michaelsen. It 

 is now perhaps the general opinion that they form 

 a family of their own, at any rate since the discovery 

 of two other genera Phreodriloides and Astacopsi- 

 drilus, besides the original genus founded by myself, 

 and named Phreodrilus from the fact that the species 

 was found in a deep well in New Zealand. 



The distribution of this family especially of the 

 genus Phreodrilus is extremely interesting. The 

 genus Phreodrilus is the only one genus of the 

 aquatic Oligochaeta which has, like Notiodrilus, a 

 circumpolar range, the pole being the south pole. 

 It occurs in New Zealand, in Kerguelen, and, if we 

 are to accept the opinions of Drs Michaelsen and 



