Earthworm: external structure, nervous system and development
The annelid body is divided into many rings; they have developed digestive, excretory, circulatory, nervous and reproductive systems. The phylum includes the classes polychaetes, oligochaetes and leeches. The earthworm is an oligochaete living in damp humus-rich soil, 8–10 cm long; each segment has four pairs of short bristles (mainly on the underside and sides) that give support in movement. The body wall is a skin-muscle sac: the skin epithelium secretes mucus that keeps it moist, and under it lie circular and longitudinal muscles. The worm thins its front end and pushes it into the soil, then pulls its rear part forward with longitudinal muscles. The body cavity is divided into chambers by cross partitions. The nervous system consists of supra- and sub-pharyngeal ganglia, a nerve ring and a ventral nerve cord; there are no special sense organs, stimuli are sensed through the skin, and complex reflexes form. The worm is a hermaphrodite: two worms fertilise each other, a cocoon forms from the girdle’s mucus, and eggs go into the soil. It can also regenerate.
On a paper strip draw 8 rings and add four pairs of bristles (dots) to each. Show the movement: when circular muscles contract the body gets thinner and longer; when longitudinal muscles contract it gets shorter and thicker. Safety: handle live worms gently, return them to the soil afterwards and wash your hands with soap.