CH. l] EARTHWORMS AND LEECHES 23 



situations sexual maturity is reached. The birds no doubt 

 get infected by devouring the earthworms: how these 

 in their turn get the parasite is not known, but it is most 

 probably by eating with the soil some of the disjecta of 

 the bird 1 . 



Within the male genital organs, imbedded in the 

 sperm mother-cells, is generally to be found the parasitic 

 Gregarine Protozoon Monocystis, whose spore-laden cysts 

 so frequently occur in hundreds in the cavities of the 

 seminal vesicles. The spores (chlamydospores) probably 

 pass with the spermatozoa into the cocoon, but how the 

 fresh, generation gains access to the worm is not known 

 with certainty. It is noteworthy, as bearing alike on the 

 life-history of this parasite and the reproductive processes 

 in the worm itself, that the chlamydospores are found 

 in the spermathecse. 



Economics. The effects produced on the surface soil 

 by the action of earthworms have been most fully pointed 

 out by Charles Darwin in his well-known book Vegetable 

 Mould and Earthworms. It will be sufficient here to 

 call attention to a few facts only. Worms play a most 

 important part in maintaining the soil in a state suitable 

 to vegetation. The burrows form ventilating tubes where- 

 by the soil is aerated and respiration by the roots of plants 

 rendered possible ; at the same time they open up drainage 

 channels, preventing the surface from becoming water- 

 logged. Doubtless also roots find an easy passage through 

 the soil along the lines of burrows even after the walls 



1 Shipley, Arch. d. Parasit. vi. No. 4, 1902, where other references 

 are given. 



