MICRODRILI 377 



genera, Phreoryctes and Pelodrilus. The former is widely spread, 

 occurring in Europe, North America, and New Zealand. Pelo- 

 drilus is limited to New Zealand. Most species of Phreoryctes are 

 distinguished by their extraordinary length and thinness, and 

 there is frequently a tendency to the disappearance of the chaetae. 

 The most important anatomical fact about Phreoryctes (at any 

 rate P. smithii) is that there are two pairs of ovaries as well as 

 two pairs of testes, and that the ducts of all are simple and very 

 much alike. This seems to argue the low position of the family 

 in the series. 



Fam. 5. Naiclomorpha} This family contains eight or nine 

 genera, perhaps more ; they are all of them aquatic and of small 

 size, and they multiply by fission as well as sexually. The most 

 noticeable peculiarity of the family is the " cephalisation " which 

 occurs in the head segments. In some genera, in Pristina for 

 example, there is no such cephalisation to be observed; but in 

 others the dorsal bundles of chaetae commence a few segments 

 farther back than the ventral, the segment where they commence 

 being different and characteristic in the various genera. Thus in 

 Dero the first four segments are without dorsal chaetae, and in 

 Nais the first five are in this condition. There is thus a kind 

 of " head " formed, whence the expression " cephalisation." Dero, 

 Nais, and Pristina are commonly to be met with in ponds, lakes, 

 etc., in this country. Bohemilla is rarer, and is to be dis- 

 tinguished by the remarkable serrated chaetae of the dorsal 

 bundles. Of Dero and Nais there are a considerable number 

 of different species ; indeed it is usual perhaps to regard as 

 distributable among three genera, Nais, Stylaria, and Slavina, 

 the species which I am disposed to place in one genus, Nais. 

 Stylaria is defined on this view by its extremely long prostomium, 

 which has given rise to both its popular and technical names. 

 " Die gezungelte Naide " was the term applied by one of its 

 earliest investigators, and the name Stylaria proboscidea signifies 

 the same peculiarity. But as the same inordinately long " pro- 

 boscis " occurs in the South American Pristina proboscidea, 

 belonging to a genus of which the other member does not possess 

 so well developed a prostomium, it seems too variable a character 

 upon which to differentiate a genus. Chaetogaster and Amphi- 



1 A. G. Bourne, "On the Naidiform Oligochaeta," Quart. J. Micr. Sci. xxxii. 

 1891, p. 335. 



