FIELD BOOK OF PONDS AND STREAMS 



the ooze of pond bottoms as thoroughly as earthworms over- 

 turn the topsoil of the land. Their slender, cylindrical 

 bodies are divided into segments set off on the inside by 

 muscular partitions, and on the outside by clearly defined 

 constrictions. They have no fleshy outgrowths along the 

 sides of the body, and no tentacles such as are found on their 

 relatives, the clamworms and the tubeworms of the seashore. 

 Their only appendages are the setae — chitinous rods which 

 prick through the skin, and are borne in varying numbers 

 upon their segments. In the common earthworm, Lumbricus 

 terrestris, such setae can be pulled down into the skin so that 

 they are partly concealed, and can be felt only when the worm 

 is stretched out at full length. All the little fresh water 

 oligochaetes, especially the family Naidce, have such con- 

 spicuous setae that the whole group of them are called bristle- 

 worms; many are transparent. Common types are Dero 

 (Fig. 113), which makes a floating tube from bits of plant 

 stems, and other naids (Naidce) which live among the latticed 

 branches of floating algse. Another family, the TuhicidcE, 

 commonly called "bloodworms," are colored blood-red and 

 are very active tube-builders on the bottom mud. 



Habitat. — Bristleworms frequent the still water of coves, 

 and stagnant, muddy, or marshy pools, where submerged and 

 decaying vegetation is plentiful. Many species hide on 

 plants in the society of snails, planarians, caddis worms, 

 and hundreds of midge lar\^as; the mud of polluted waters is 

 often literally astir with them (Fig. 19). They can live 

 with but little oxygen. Tubifex have been taken from the 

 depths of Lake Mendota, Wisconsin, in waters which are low 

 in oxygen for three months at a time. 



Food. — ]\Iost oligochaetes live on decayed organic matter, 

 especially plants; they are constantly cleaning up the pools 

 and they are largely responsible for the quick disappearance 

 of dying algal mats (p. 50) in the fall. Charles Darwin's 

 famous "Vegetable Mold and Earthworms" portrays how the 



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