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EVOLUTION OF ANIMALS 



Part V 



Fig. 28.2. Amphitrite johnstoni (8 to 10 inches long). The small scale worm 

 Gattyana cirrosa lives in the tube as a partner and takes the food that escapes the 

 larger worm. Various species of Amphitrite hide in sand-covered burrows but their 

 filamentous crimson, or crimson and yellow gills wave freely in the water. Amphi- 

 trite, named for the Greek goddess of the sea, has a near relative named for 

 Aphrodite, the goddess of beauty. (Photograph courtesy, Douglas P. Wilson, 

 Marine Biological Laboratory, Plymouth, England.) 



The developing young ones swim about freely, but the adults are nearly all 

 rocking-chair travelers moving back and forth within their tubes. For various 

 burrowers and most tube dwellers, the sticky mucus on a thrusting proboscis 

 or waving gills is a means of collecting food. Many of them live alone in hard 

 tubes of calcium carbonate fastened to rocks, to seaweeds and oyster shells. 

 The majority of those in the larger, soft tubes have one or more guests, com- 

 mensal worms and crabs that share the house and whatever board they can 

 collect. On the Atlantic coast, a little crab (Pinnixa chaetopterana) lives with 

 Chaetopterus and moves with the worm, keeping near its mouth for the extra 

 "crumbs." Annelid scale worms are frequent guests. Some of these, like 

 parasites, will live with only one kind of host; for others, almost any tube 

 will do. 



Characteristics. Annelids may be scarcely visible to the naked eye or several 

 feet in length. A seaweed feeder {Neathes brandti) of the Pacific coast of 

 North America is six feet long when relaxed and the giant earthworms of Aus- 

 tralia reach 10 and 12 feet. The characteristic structures of annelids are the 

 segmentation of the body before mentioned; a true coelom lined with a peri- 

 toneum; a central nervous system in which the brain (a pair of dorsal ganglia) 

 is connected with a double ventral nerve chain expanded in each segment 



