Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 555 



into a ganglion; and chitinous bristles or setae usually present on most seg- 

 ments. Whenever a larval stage is characteristic of the species, it is the trocho- 

 phore type similar to those of many other aquatic invertebrates (Fig. 27.1). 

 The ancestors of annelids lived in the Cambrian Period, the early part of the 

 Age of Invertebrates. 



Class Oligochaeta 



The Earthworm 



The earthworm, Lumbricus terrestris, is an immigrant from Europe that 

 spread through the eastern part of North America and, at least, in labora- 

 tories has reached the west coast, a few years ago more conspicuously than now. 



Ecology. These burrowers clear their way through the soil mainly by 

 swallowing it. In spite of a long land residence and earthy contacts inside and 

 out, the bodies of earthworms are excessively water hungry (Fig. 28.3). Their 

 skins are too permeable for real land life. A worm that is transferred to water 

 absorbs 15 per cent of its initial weight in 5 hours and then levels off, water- 

 adapted. Conversely a water-adapted worm removed to moderately dry soil 

 loses water for a few hours, then levels off, semi-land-adapted. As a conse- 

 quence of their need for water, earthworms rarely live in dry climates and are 

 active only in the rainy seasons. They benefit the soil by loosening and aerating 

 it, swallowing and carrying top soil downward and deep soil upward. Thus, 

 they have plowed the land for centuries. Charles Darwin brought out the im- 

 portance of this in his "The Formation of Vegetable Mould, through the 

 Action of Worms with Observations on their Habits" — his last book, pub- 

 lished in 1881. It is the account of observations and experiments continued for 

 over 20 years in his "earth worm field" close to Downe House, his home 

 near London. 



The Outer Tube — Protection, Locomotion, and Support. The earthworm's 

 mouth is overhung by a supple grasping lip, the prostomium. The flattened rear 

 end of the body is pressed against the inside of the burrow, a holdfast when 

 the worm is extended on the surface (Fig. 28.4). 



Earthworms are dark colored above and pale on the underside, embar- 

 rassingly good examples of counter-shading although they are strictly noc- 

 turnal. Such examples are thorns in the theory of counter-shading which is based 

 on the presence of strong light from above. The conspicuous glandular swelling 

 is the saddle or clitellum which secretes the cocoon that contains the develop- 

 ing eggs. On each segment except the first and last there are four pairs of 

 minute chitinous setae. Each seta can be moved in several directions, also 

 extended or withdrawn into the flesh, and the worms catch the ground with 

 them as they crawl. On a quiet night the sound of moving earthworms can be 

 heard among dry leaves, like sandpaper catching against the edge of paper. 



There are numerous microscopic openings in the skin, those of the mucous 



