The earthworm lays its eggs in a small cocoon formed by the hardening of a viscid material which is discharged by a swollen part of the body called the clitellum, extending from the 32nd to the 37th segments. After the formation of the cocoon the worm moves backwards, and the eggs leave the body by small pores on the ventral surface of segment 14, [Pg 387] as this region passes the cocoon. A small amount of food-material is also enclosed in the cocoon, and forms a store of nutriment for the young worms during their early development. EXERCISES ON CHAPTER XX. 1. Compare the legs of a cockroach with those of a crayfish and a vertebrate. (1900) 2. Describe the respiratory organs of the crayfish. How are they continually supplied with fresh water? (1898) 3. Examine a spider. How many legs has it? Of what divisions does its body consist? Why do you consider that a spider is not an insect? 4. Make observations, and write descriptions, of the habits of spiders, paying special attention to the methods of construction of the webs, the manner of catching prey in different cases, and the care of the young by the parents. 5. Compare a centipede with an insect, pointing out the features of resemblance and difference. (1897) 6. How does a pond-mussel open and close its shell? (1900) 7. Compare an oyster with a fresh-water mussel, and try to find points of resemblance and difference, making careful notes and sketches of these. How many closing muscles has the oyster? Are its gills plate-like? Why do you consider the oyster a mollusc? 8. Where is the lung of a snail situated? How do we know that it is really a lung? (1897) 9. How does an earthworm resemble and differ from a caterpillar? (1900) 10. Explain the action of an earthworm in the formation of vegetable mould. (1897) 11. Classify the animal described below: A land animal with a long narrow soft body and no legs; two pairs of tentacles on the head; a breathing hole on one side? (1904) 12. Give the characters by which insects can be distinguished from crustaceans. (1905) [Pg 388] CHAPTER XXI. FIELD-WORK.