The large burrowing terebellid worm at left, Amphi- 

 trite johnstoni, shares its burrow, and probably a 

 h'ttle of its food, with a smaller guest ( commensal ) , 

 the scaleworm Cattyana cirrosa. (England. D. P. 

 Wilson ) 



To retreat, the worm simply tilts its bristles in the 

 opposite direction and repeats the cycles of changes 

 in body length, time after time. 



This method of locomotion is adequate for oligo- 

 chaetes wherever they live. A very few, such as the 

 very slender, 2'/i-inch Clitellio worms found under 

 stones in the intertidal zone, and the white, 1-inch 

 Enchytraeus worms among living and dead plant de- 

 bris in the same regions, can tolerate salt water. A 

 few creep over glaciers, apparently feeding on micro- 

 scopic plants in the film of water that melts when sun 

 reaches the ice surface. These are "glacier worms." 

 But most oligochaetes live in fresh water or in soil, 

 on all continents and major islands with the exception 

 of Antarctica and Madagascar. 



Oligochaetes in fresh water inhabit their realm 

 from top to bottom, and tend to be world-wide in dis- 

 tribution, perhaps carried from place to place in the 



mud on the feet of migrant wading birds. Little Dero, 

 scarcely V4 of an inch in length, secretes a slender 

 tube below a duckweed leaf, and slips in and out (or 

 reverses itself) at astonishing speed. Dero captures 

 small crustaceans (chiefly water fleas) that come to 

 rest against the undersurface of the miniature floating 

 plant. 



Bright red or brownish members of the genus Tii- 

 bifex thrive where the bottom mud is rich in organic 

 matter, and can manage with almost no oxygen. Of- 

 ten they become conspicuous as an assemblage form- 

 ing a carpet in polluted water. If undisturbed, each 

 worm clings to the upper end of its individual slime 

 tube, a tunnel extending downward into the bottom. 

 Its inch-long slender body waves in the water, creat- 

 ing a feeding current. Yet the merest commotion is 

 enough to send every Tiibifex instantly into its refuge. 



Many fresh-water oligochaetes reproduce asexu- 

 ally, simply by dividing the body transversely into 

 two, each half regenerating the missing region. Often 

 the regeneration is essentially complete before the in- 

 dividuals separate. Sometimes several offspring of 

 this kind are being formed at the same time, one af- 

 ter the next in a chain. 



Otherwise the bristle-footed worms mate in the 

 manner that has become familiar from observations 

 of earthworms. Each oligochaete is both male and fe- 

 male. At mating time a pair of equal size lie with an- 

 terior ends overlapping, facing in opposite direc- 

 tions. Sperm cells are passed from each worm to the 

 other, and collected in special flask-shaped chambers 

 opening to the outside by small openings. Then the 

 worms separate, each to use the store of sperms in 

 fertilizing a batch of eggs. 



When the eggs are ready, the worm secretes a 

 slime tube around its body, heaviest and charged 

 with a special protein in the region of the clitellum 

 (the saddle-shaped thickening so conspicuous a short 

 distance back from the head end ) . Slowly the worm 

 moves in reverse, sliding the slime tube like a girdle 

 over the anterior end. As the thick part of the tube 

 passes the sperm cavities, these discharge their load 

 into the space between the worm and the slime tube. 

 Opposite the ovaries, the eggs are extruded into the 

 same space and fertilization occurs. Gradually the 

 worm works out of the slime tube, and the latter 

 purses up to form a lemon-shaped cocoon enclosing 

 the developing young. By the time they escape from 

 the cocoon, they are miniatures of the parent. 



Quite recently, scientists have discovered that the 

 common earthworms emerge from the cocoon with 

 the full adult number of segments. In Lumhriciis ter- 

 restris, the flat-tailed night crawler, this is about 150. 

 In the barnyard earthworm Eisenia foetidci, which 

 Izaak Walton called the "brandling," the total is only 

 about 95 segments. 



The clitellum, too, has a definite position. That of 



206] 



