32 EARTHWORMS AND THEIR ALLIES [ch. 



Other internal organs show certain points of 

 likeness in all or in the great majority of the aquatic 

 families. Thus the nephridia are without a plexus 

 of blood capillaries surrounding them, a state of 

 affairs which also occurs in some of the slender 

 Ocnerodrilinae among the earthworms. These paired 

 organs also are very frequently not found in the anterior 

 segments of the body and these include also as a 

 general rule the segments in which the reproductive 

 elements are formed. Save for an exceptional case 

 among the genera of Enchytraeidae the dorsal pores 

 are not found among the Limicolae, but in some cases 

 at least a single pore, the head pore, is found. The 

 sperm ducts, which among earthworms usually (and 

 indeed always save in the anomalous Moniligastridae) 

 traverse a considerable number of segments on their 

 way from the internal opening into the body-cavity 

 to the external pore, do not show the same pheno- 

 menon among the Limicolous Oligochaeta. They are 

 sometimes indeed limited to a single segment, that 

 is to say the funnels and the external pore lie in one 

 segment. In other cases they open on to the exterior 

 in the segment next to that which bears the funnel, 

 and it is only rarely that they traverse more than 

 one segment. There are also points of difference of 

 general applicability to be noted in the sperm sacs 

 and egg sacs. The latter are large and extensive, 

 which is not the case among earthworms, and the 



