Earthworm Morphology

Abstract

Earthworms are bilaterally symmetrical, externally segmented, with a corresponding internal segmentation. They have no skeleton and a thinly pigmented cuticle, bearing setae on all segments except the first two, with an outer layer of circular muscles and an inner layer of longitudinal muscles. Earthworms are hermaphrodite and have relatively few gonads, situated in consistent segmental positions. When mature, and after fertilization of the eggs, a swollen area of the epidermis termed a clitellum, located in particular segments, forms a cocoon in which the eggs or ova are deposited, and this is then passed over the anterior segment. The eggs are usually fertilized as the clitellum passes along the body and is extruded as over the head a cocoon and the young develop within the eggs without a free larval stage, the newly hatched worms resembling adults. A few species can reproduce occasionally or consistently parthenogenetically.