Clitellata is a class of Annelid worms, characterized by having a clitellum - a collar that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species. All clitellata are hermaphrodites. During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygotes or both ova and sperm into the coat, which is then packed into a cocoon. The zygotes then evolve further directly in the cocoon without passing through a larva stadium (as opposed to other annelids, e.g. polychaeta.) Description accessed from WikiPedia 3rd August 2008, and modified.
"[S]oil does not seem to adhere to earthworm surface…An important phenomenon of earthworms moving in moist soil is that the electric potential exists on an earthworm tissue…The electroosmotic flow near an earthworm body surface is a basic electrokinetic phenomenon that takes place when the earthworm moves in moist soil. The flow in a micro thin layer of water is formed in the vicinity of the earthworm body surface as a result of the electric double layer (EDL) interaction. Such a micro scale electroosmotic flow plays the role of lubrication between the earthworm body surface and the surrounding medium of moist soil and reduces surface adhesion." (Zu and Yan 2006:179) Learn more about this functional adaptation.
Zu YQ; Yan YY. 2006. Numerical simulation of electroosmotic flow near earthworm surface. Journal of Bionic Engineering. 3: 179-186.
"The basic worm trick consists of stretching and squeezing alternate parts of a long, cylindrical trunk, moving each region of stretch or squeeze rearward, as in figure 24.7a. Despite the rearward progression, the scheme can't do much without one more component. Thus, a worm on a smooth and lubricated surface makes negligible progress--we're not looking at an analog of anguilliform (eel-like) swimming. The trunk needs some device so it slides more easily forward than rearward. For earthworms, setae, short rearward-pointing bristles, provide that crucial asymmetry." (Vogel 2003:488) Learn more about this functional adaptation.
Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:25383 Specimens with Sequences:18563 Specimens with Barcodes:17487 Species:1755 Species With Barcodes:1540 Public Records:15726 Public Species:1009 Public BINs:3297