DESCRIPTIONS OF GENERA AND SPECIES 311 



MAEIOKTA. Setae /-shaped ; uo dorsal pores and other characters as in "Stf 



Pachydrilus, except that testes are massive, not subdivided. 

 BBYODHILUS. Setae /-shaped ; no dorsal pores ; dorsal vessel arising behind JZ 



oesophageal glands in xii ; blood colourless. 

 PABENCHYTHAEtrs. Setae straight ; no dorsal pores ; ventral vessel bifurcates only 3>>'j 



in first segment. 



The arrangement of the genera of this family naturally depends upon the affinities of 

 the family as a whole to other Oligochaeta. Regarding, as I believe it to be 

 necessary to do, the more simple forms as standing higher or lower in the scale 

 (according to the sense attached to the words) than the more complexly-organized 

 genera, in fact, as more specialized through degeneration, it appears to me that 

 the genus Mexenchytraeus represents, on the whole, the most primitive Enchytraeid. 

 I base this view of its position on the following characters : 



(1) The setae are sigmoid in shape. 



(2) There are sperm-sacs and egg-sacs. 



On the other hand, the colourless blood, the absence of dorsal pores, and the 

 rudimentary condition of the oesophageal glands (represented only by the cardiac 

 body), furnish evidence against the placing of the genus. 



The genus Buchholzia exhibits two of these last characters with the addition of 

 the sigmoid setae ; it has, however, colourless blood like Mesenchytraeus. Pachydrilus 

 has coloured blood and sigmoid setae. How are wo to decide between the conflicting 

 claims of these three forms ? 



A fourth, indeed, might be added, viz. Fridericia; this genus has dorsal pores, 

 a character distinctive of the higher Oligochaeta. 



It seems to me that the importance of these several characters is indicated by 

 their position in the following list: 



(1) Sperm-sacs and egg-sacs. 



(2) Oesophageal glands 

 Sigmoid setae 



(3) Red blood. 



(4) Dorsal pores. 



(1) Sperm-sacs are, with the exception of certain Enchytraeidae (the majority), 

 present in all Oligochaeta, even in the lowly form Aeolosoma. They must evidently, 

 therefore, be looked upon as being very distinctive characters of these Annelids. 



(2) Oesophageal glands are met with in none of the lower aquatic Oligochaeta, 

 excepting certain Enchytraeidae. They are very common in the higher forms, 

 occurring in nearly all the ' earthworms.' Their presence or absence, therefore, 



| (equal). 



