By C.A. A Edwards
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IIa Nephrostome -ventral view. [After Grave and Newell, 1962 (based on Goodrich)] Centra l cell Lower lip Pentoneum \ / narrow tube Fig. llb Nephrostome - longitudinal section. [After Grave and N ewell, 1962 (based on Goodrich)] 26 BIOLOGY OF EARTHWORMS and inner faces, the cilia beating towards the canal. The lip is peripheral to a large crescent-shaped central cell with nucleate marginal cells and cilia on its lower face. The dorsal wall of the preseptal canal extends to the lower boundary of the central cell, forming a part of the upper lip which has no nuclei or cilia, neither has the lower lip which is much thinner than the upper lip.
However, the most important family in terms of human welfare is undoubtedly the Lumbricidae, the most recently evolved family. This family of worms is of particular importance because it is the dominant endemic family in the Palaearctic zone including Europe, where until recent times many advances in agricultural practice have originated. Because of their ability to colonize new soils and become dominant to the near exclusion of local endemic species, the Lumbricidae have followed the spread of civilization, colonization and human development around the world.
The clitellum consists of thickened glandular epidermis, particularly on its dorsal and lateral portions. A section through the clitellum (Fig. 17) shows gland cells in three layers; those nearest the surface are mucous cells, which are similar to the mucussecreting goblet cells in the ordinary epidermis. Also reaching the surface, but extending deeper into the clitellar tissue, are long, slender and often convoluted cells, containing large granules. The bulk of the clitellar tissue in the deeper layer down to the basal membrane is made up of albumen-secreting gland cells terminating distally in long slender ducts which open on to the body surface.