224 



GENERAL ZOOLOGY 





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

 well to many genera of slender animals that live continuously 

 in tubes of mud, sand, or limy secretions (Ser'pula, in Fig. 77) 

 in the sea. In an animal remaining fixed in the sand, like 

 Amphitri'te orna'ta (Fig. 108), it would seem to be economy in 

 animal architecture not to have broad swimming appendages 

 like those of its relative, Nereis. On the same principle we 

 observe that gills (Fig. 108, l) occur only at the anterior end 



where they may obtain oxygen from 

 the circulating water above the 

 animal. To counterbalance the dis- 

 advantage of a fixed habitat, Am- 

 phitrite has many long tentacles 

 (Fig. 108, 2) 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 constantly toward the ani- 

 mal's mouth, carrying in the micro- 

 scopic food present 

 in the water. At 

 the same time the 

 bristles, aided by 

 mucus which is se- 

 creted from glands 

 on the tentacles, 

 catch up grains of 

 sand. These are 

 added to the little 

 hummock that con- 

 ceals the mouth of Arnphitrite's permanent home. 



The Leech. Most species of leeches are known to be tem- 

 porary parasites on other animals, but Placobdel'la rugo'sa 



FIG. 109. Leech. Slightly reduced 



