350 APPLIED BIOLOGY 



tains decaying organic matter, crawls out at night to feed, 

 or when its burrows are flooded with water. It eats the soil 

 through which it burrows, and its digestive juices dissolve 

 bacteria, leaf -mold, and other organic matter contained in 

 the soil. The indigestible soil is discharged and forms the 

 " castings " which are abundant on the surface of soil where 

 the animals live. Darwin found places where he estimated 

 that the earthworms brought more than 35,000 pounds of 

 soil per acre to the surface in a year. This continual work- 

 ing of soil by worms is regarded as of great agricultural value. 

 See Darwin's " Vegetable Mold and Earthworms." 



General External Structure. (L) Notice a living worm as it moves, 

 whicn if doesTby successively elongating and shortening its body. 

 Determine anterior, posterior, dorsal, and ventral. Compare the 

 color on the dorsal and ventral surfaces. Is the animal bilaterally 

 symmetrical ? Is there a head, thorax, and abdomen, as in the frog ? 

 Estimate the number of segments or rings in the body. Locate the 

 mouth and arms. Notice the glistening surface of the skin due to a 

 cuticle. About one third or one quarter the body length from an- 

 terior end, there is a swollen region, the girdle or clitellum. It con- 

 tains gland cells which secrete a cocoon in which the eggs are laid 

 ( 303). The clitellum is not present on very young worms and on 

 older ones only in spring of the year, the egg-laying season. Four 

 double rows of small bristles (setae), which aid the worm in locomo- 

 tion, may be located by pulling a preserved specimen between a 

 thumb and a finger, and by using a hand-lens. How many bristles 

 are on a segment? Make dia rams showing their position. 



Living earthworms should be observed with regard to (1) move- 

 ments on soil and on hard surfaces, and (2) reactions to light and 

 touch. 



The dorsal and ventral blood-vessels may be easily seen in a 

 living animal. Movements of the dorsal blood-vessels can be easily 

 seen if a light-colored worm is selected, placed on a moist glass plate, 

 and another piece of moist glass then placed on top of the worm so 

 as to compress the body slightly. 



The Body-cavity. (D) Split a preserved earthworm lengthwise 

 into right and left halves and note that the body-cavity (coelome) 

 is divided by partitions into cavities corresponding to the external 

 division into segments. Examine the body-wall and the alimentary 



