APPENDIX B 



647 



Triploblastic ; bilateral symmetry; metamerism well developed, each 

 metamere (somite or segment) more or less similar to the others. Blood 

 vessels, excretory organs (nephridia), and nervous system segmentally 

 arranged. Tube-within-tube type of organization; alimentary canal dif- 

 ferentiated into organs; distinct coelom present, lined with mesoderm 

 and often divided into chambers by transverse metameric partitions. 



A complex circulatory system present, operating with respiratory organs (ex- 

 ternal gills) in some of the marine Annelida (Fig. B.16); ventral double nerve 

 cord with nerve centers (ganglia) in each metamere, constituting a "ladder type" 

 of central nervous sj^stem, in front encircling the alimentary canal to form a 



Fig. B.16. The plankton-straining marine annelid Amphitrite. The gill-bearing heads of two 

 individuals are seen protruding from their tubes; a third is partly retracted. Encrusting 

 bryozoan colonies are also shown. {Courtesy American Museum of Natural History.) 



"brain" above it. When present, the appendages are not jointed or segmented but 

 in the form of spines (earthworms), fleshly flaps (marine Annelida), or suckers 

 (leeches). 



Most annelids are marine, but many live in fresh water or in the soil or other 

 moist places. A few are parasitic, notably the leeches, many of which are external 

 parasites on the bodies of aquatic animals and a few on terrestrial animals. 



The common earthworm (Lumbricus, Fig. 13.7 and 15.6) is the most frequently 

 studied annelid. Earthworms are soft-bodied and "slimy." They live in moist 

 earth and venture out of their burrows chiefly on damp nights. The burrows 

 usually extend about 2 feet beneath the surface. The worms can force their way 

 through soft ground but must eat their way through harder soil. The earth eaten 

 passes through the alimentary canal and is deposited on the surface of the ground 

 as "castings." Decaying vegetable matter in the soil provides food; the worms also 

 eat leaves and other surface vegetation, which they drag into their burrows at 

 night. Thus earthworms continually honeycomb the soil, making it more porous, 

 permitting better penetration of air and moisture, and increasing its fertility. 



