The external features of the native earthworms are illustrated in the following diagram. Their size is extremely varied; the largest, Spenceriella gigantea from North Auckland, attains a length of 4 ft 6 in. and a diameter approaching ½ in., while a number of the smaller species are from ½ in. to 1 in. long and less than 1/10 in. in diameter. Most of the native species are red or brown. The red colour is sometimes due entirely to the haemoglobin of the blood showing through the epidermis, but red, purple, or brown pigments are frequently present on the dorsal surface, the ventral surface being usually unpigmented or very lightly so. Certain species, especially those that inhabit forest leaf mould, are deeply pigmented, some with striking patterns of contrasting colours, and these species are usually pigmented both dorsally and ventrally.

The body of the earthworm is divided into segments which are separated externally by intersegmental furrows that correspond to internal septae. Minute chaetae, or bristles, used in locomotion, occur on each segment except the first, each segment having 8, 10, 12, or more. Commencing on or in front of the fifteenth segment is the clitellum, a thick glandular portion of the epidermis, developed over a number of segments on sexually mature worms in the region of the male and female pores. Earthworms are hermaphroditic, both male and female organs being present, but they are not self-fertilising and, when mating takes place, sperm cells are exchanged. During oviposition the clitellum secretes a gelatinous sleeve-like structure, worked forward over the body, which receives ova as it passes the female pores and sperm cells as it passes the spermathecal pores. As the “sleeve” passes off the anterior end of the body, its ends are sealed and it becomes a cocoon.

Native earthworms feed almost entirely on dead and decaying remains of plants and, because of their limited capacity to move about, they are obliged to live very close to their sources of food. The presence of free water is essential for they have virtually no mechanism for conserving moisture. Respiration takes place by diffusion of gases through the moist body wall; hence both moisture and dissolved oxygen are essential. Earthworms are injured and may die by exposure to daylight, except when the intensity is very low, the more pigmented species being more resistant to light damage than the less pigmented. They are killed by temperatures in excess of 85°F–100F, but in most New Zealand habitats they escape the effects of extreme high or low temperatures by retreating to lower layers.

The pH tolerance varies from species to species but no native earthworms have been found in soils lower than pH4. Most earthworms are able to tolerate submersion in water and there are a few species that prefer an aquatic life. During heavy rains, however, earthworms are commonly driven to the surface, but this is most probably due to the shortage of oxygen in the water in their burrows.