254 METAZOAN PHYLA 



The larva of most of the marine anneUds is a trochophore (Fig. 153 A 

 and B). This larva does not occur in fresh-water forms and of course is 

 never seen in the terrestrial types. 



291. Occurrence and Economic Importance. — The distribution of 

 various anneUds, especially the marine forms, is very general, though 

 characteristic species are found in each region. Economically the earth- 

 worm has been shown to be generally a very beneficial type. The 



Asexua/ 

 zooid 



Sexual 

 200 ic^ 



Fig. 160. — -A palolo worm, Leodice viridis (Gray), showing asexual and sexual zooids. 

 {From VanCleave, "Invertebrate Zoology," after Woodworth, by the courtesy of McGraw-Hill 

 Book Company, Inc.) 



leeches, which live a parasitic life, should be considered injurious, enemies 

 of both man and the domestic animals which serve him. The medicinal 

 leech, however, has in the past been of service to physicians and played 

 such a part in medicine in the sixteenth century as to earn for the phy- 

 sician himself the appellation of leech. Most other annelids are of little 

 importance, though some are used as fish bait. In the South Seas the 

 palolo worm (Fig. 160) is gathered by the natives when it swarms in the 

 breeding season in October and November and is used as food, being 

 considered a great delicacy. 



