58 EARTHWORMS AND THEIR ALLIES [ch. 



simple structure of the terminal gland into which 

 they open and which in its turn opens on to the 

 exterior is very like that of such a family as that of the 

 Lumbriculidae. Another fact is the simple undivided 

 cavity of the sperm sacs which is unlike that of 

 typical earthworms but again like that of all of the 

 Limicolous families. We may fairly see in these 

 worms evidence of origin from aquatic ancestors. 

 Evidence of the same nature, i.e. not as showing the 

 retention more or less of anatomical characters 

 commonly associated with a life in water, but as 

 affording indirect evidence of an origin from actually 

 aquatic forms, is to be seen in certain members of 

 the families Geoscolecidae and Eudrilidae. In both 

 of these it not infrequently happens that the sperm 

 sacs are but a single pair and that that pair consists 

 of sacs of extraordinary length. Thus in Trichochaeta 

 (or Hespe7'osGolex)harhadensis Miss Fedarb and I have 

 shown that the long thin sperm sacs extend through 

 no less than 109 segments, which is vastly in excess 

 of the length of those of the majority of earthworms 

 in which they are most commonly limited to a single 

 segment. In the same way the Eudrilid worm Poly- 

 toreutus magilensis has a pair of long and thin sperm 

 sacs which extend through some fifty segments. This 

 elongation of the sperm sacs in the ripe worms is a 

 very common feature of the Limicolous genera. 



