The Pheretima (Earthworm) Type Study

The Pheretima (Earthworm) Type Study

Pheretima, the common Indian earthworm, is studied in detail as the representative type of class Oligochaeta and phylum Annelida, because its organ systems illustrate the class's features very clearly.

Nephridia are the excretory organs of Pheretima, present in large numbers, repeated segmentally along the body. Each nephridium is a coiled tubule that collects coelomic fluid and waste (chiefly nitrogenous waste) and discharges it to the exterior, functioning much like the flame-cell system of flatworms but organised as a much more elaborate, segmentally repeated tubular structure appropriate to a coelomate animal. Pheretima's nephridia are of three types, based on their position and where they open: septal nephridia, embedded in the septa (partitions) between segments and opening into the gut, so their waste is voided along with the faeces; pharyngeal nephridia, clustered around the pharynx and oesophagus region and also opening into the gut; and integumentary (or exonephric) nephridia, attached to the inner body wall and opening directly to the exterior through the skin, independent of the gut. Between them, these three types drain waste from essentially every part of the earthworm's segmented body.

Blood glands are paired glandular masses found in a few specific anterior segments of Pheretima, closely associated with the blood vessels; they are believed to manufacture blood cells and haemoglobin, effectively functioning as a blood-forming (haemopoietic) tissue for the animal, broadly comparable in role to bone marrow in vertebrates. …