70 



BRANCH ANNULATA 



soil. The eggs are hatched in four or five weeks, but it takes 

 them several years to mature. Some leeches are said to live 

 twenty years. 



Leeches are widely distributed. Many of them are in- 

 habitants of fresh water. Some live in salt water, while others 

 live in the forests of many regions, especially those of the 



tropics, where they are the terror of men 



and beasts. 



One species ( Hiru'do sanguisu'ga) is a parasite 

 in the nasal passages of man. Another ( Hcemop'- 

 sis vo'rax) lives in the pharynx or trachea of the 

 horse, being taken in with water when small. 

 Another form (Branchel'lion} is a permanent ex- 

 ternal parasite on fishes. 



'\,'S 



Distribution. The members of branch 

 Annulata are widely distributed, the rep- 

 resentatives of its many species being 

 found from frigid to tropical regions, and 

 even in the isolated islands of the sea. 



It is known that marine worms existed 

 in the Cambrian Period by their " tracks 

 and borings in the sand, which are now 

 consolidated into hard rocks." 



Economic Importance. The earth- 

 worm swallows the soil which it exca- 

 vates for the sake of the partially de- 

 cayed organic matter it contains, which 

 the worm appropriates to the building 

 up of its body tissue. The indigestible 

 portions it deposits on the surface at 

 night as coiled castings. They also feed 

 on fresh or decayed leaves which they 

 drag into their burrows, and sometimes 

 upon young seedlings and tender roots. 



Darwin, who studied the earthworm for 

 forty years, estimated that in the tillable 

 soil of England there were fifty thousand 



earthworms to the acre, and that they brought to the surface 

 from 10 to 18 tons of soil annually. In this way the whole 



Fig. 51. Section of 

 a leech: a, Anterior 

 sucker; b, posterior 

 sucker; c, anus; d, d, d, 

 stomach ; ce, esophagus ; 

 i, intestine; s, s, glands 

 of the skin. (Holder.) 



