374 



OLIGOCHAETA 



CHAP. 



to the clitellum, and in this case it is not so easy to decide 

 their systematic position. 



The Megadrili are characterised by the precisely opposite 

 characters. The sperm-ducts are longer; the clitellum is com- 

 posed of many layers of cells ; the egg-sacs are rudimentary ; 

 sexual maturity appears to be more or less continuous. 



There is, however, a substantial agreement about the families 

 which I here adopt, which may be fairly taken to express our 

 present knowledge of the Order. For fuller details the reader is 

 referred to my Monograph of the Order Oligochaeta. 1 



I. Microdrili. 



FAM. 1. Aphaneura. 2 This name was originally given to the 

 present family by Vejdovsky ; the family contains a single genus, 

 Aeolosoma, of which there are some seven species. The name is 



taken from, perhaps, the 

 most important though 

 not the most salient 

 characteristic of the 

 worms. The central 

 nervous system appears 

 in all of them to be re- 

 duced to the cerebral 

 ganglia, which, moreover, 

 retain the embryonic 

 connexion with the epi- 

 dermis. The worms of 

 the genus are fairly 

 common in fresh waters 

 of this country, and they 

 have been also met with 

 in North and South 

 America, and in Egypt, 

 India, America, and 

 tropical Africa. They 

 are all small, generally 

 minute (1 to 2 mm. long), and have a transparent body variously 

 ornamented by brightly-coloured oil globules secreted by the 

 1 Oxford, 1895. 2 See especially Vejdovsky, Syst. u. Morph. Olig. Prag, 1884. 



Fig. 195. Aeolosoma hernprichii dividing transversely 

 x 30. (After Lankester.) 



