252 



GENERAL ZOOLOGY 



same division into somites of approximately uniform struc- 

 ture throughout the body. Fully developed locomotor 

 organs, and a distinct head with eyes and tactile sense 

 organs, serve to adapt Nereis to its environment. 



The Tube Worm. The name "tube worm " applies equally 

 well to many genera of slender animals in the sea that live 



i 



Fig. 131. Living sandworm. ^Reduced) 



continuously in tubes of mud, sand, or limy secretions. In 

 Amphitri'te orna'ta (Fig. 132) the gills occur only at the 

 anterior end, where they may obtain oxygen from the cir- 

 culating water above the animal. To counterbalance the 

 disadvantage of a fixed habitat, Amphitrite has many long 

 tentacles which extend out over an area sometimes of three 

 square feet. These tentacles are covered with scattered, fine 

 bristles and with many cilia; the cilia are waving con- 

 stantly toward the animal's mouth, carrying in the micro- 

 scopic food present in the water. At the same time the 

 bristles, aided by mucus which is secreted from glands on 



