INVERTEBRATA. 



343 



especially abundant in the head-region and around the 

 setae, are globular aggregations of the sensory cells already 

 mentioned. These are probably concerned mainly in tactile 

 sensibility, but we know that the earthworm is sensitive to 

 light, perhaps also to sound and smell. 



8. Reproduction. Like Hydra, the earthworm is herma- 

 phrodite, but unlike it, is incapable of self-fertilization, there 

 being elaborate arrangements for mutual cross-fertilization 

 between two 

 worms. The 

 gonads, as in 

 Vertebrates, are 

 derived from the 

 coalomic epithe- 

 lium : in many 

 members of the 

 group to which 

 Lumbricus be- 

 longs, they are 

 metameric (as in 

 Amphioxus), but 

 in Lumbricus it- 

 self only a trace 

 of this is seen, 

 there being two 

 pairs of testes, 

 and one pair of 

 ovaries. The 

 former are at- 

 tached to the septa at the front end of somites 10 and 11 ; 

 the latter occupy a similar position in somite 13 (fig. 178). 

 In Vertebrata we have seen that the oviduct has a free 

 internal opening into the ccelom, whereas the male duct 

 is directly continuous with the tubules of the testis. But 

 in Lumbricus both male and female ducts open freely into 

 the coelom, by funnel-shaped ends lying a little behind each 

 gonad. The two vasa deferentia of either side unite into 

 a pair of longitudinal ducts which run back to their external 

 openings on the ventral side of somite 15 : these apertures 



-t*C*YC C0#> 



/S 



Fig. 178. REPRODUCTIVE ORGANS OF EARTHWORM. 



Dissection from dorsal side. (After Marshall and Hurst, 

 altered.) For " Spermathecae " read " Spermothecse." 



