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What do gardeners often find when they attempt to dig up soil in order to plant vegetables? It is most likely that they would find earthworms in the soil. As a matter of fact, according to scientists, there are approximately eight million earthworms, or night crawlers, per acre of moist soil. The earthworm, whose scientific name is Lumbricus sp., is a member of the phylum Annelida and the class Oligochaeta, a class that approximately consists of about seven thousand species. The phylum Annelida belongs to the domain of protosome coelomates, which also include such phyla as the phylum Mollusca and the phylum Arthropoda. Being protosome coelomates, they all possess a fluid-filled body cavity, which may too serve as a hydrostatic skeleton against which muscles in the body wall can contract to produce movement. The phylum Mollusca includes such familiar creatures as clams, oysters, snails, and octopi. Even though there is a tremendous diversity amongst all seven classes of the phylum Mollusca, they all feed with help from a rasping tongue called a radula, a tiny little chainsaw-like structure made of chitin. The phylum Arthropoda consists of five classes—most of the members in these five classes possess a protective exoskeleton that is made out of chitin. (Fried and Hademenos, 1999) The common representatives of the phylum Arthropoda include spiders, insects, and crustaceans. Nonetheless, the phylum Annelida is distinctly diverse from the other phyla in the protostome coelomates domain due to their segmentation. In addition to the class Oligochaeta, the phylum Annelida also includes the class Polychaeta, which mostly comprises marine worms, and the class Hirudinea, which mainly consists of leeches. All members of the three classes in the phylum Annelida characteristically have a body that becomes organized into a linear series of identical compartments called segments. Segments allow annelids to move efficiently over solid surfaces; moreover, since all of the segments are identical, if one segment is damaged, the others may be able to survive and repair the damage. All forms of the phylum Annelida demonstrate an excretory system that functions by means of a pair of excretory tubules called metanephridia located in each segment that are connected to nephrostomes, which are tunnels that picks up fluid from the coelom. The metanephridia lead to the nephridiopores, pores that excrete wastes to the outside. Despite sharing numerous physiological similarities with other classes in the phylum Annelida, the class Oligochaeta, or the earthworms, is undoubtedly the most known representative due to its prevalent role as being common subjects in biology teaching and researches.

(Photograph by Jack Kelly Clark, 2003) This is an image of a live earthworm. Earthworms are present all over the world. However, they are often found in deep, dark, narrow, and moist underground burrows. They are called as night crawlers due to the fact that they could not tolerate heat and sun so they come up to the surface only at night whenever the weather is too hot during the day. It is usually common to see a great number of earthworms on the surface after a rain because the wet ground allows the earthworms to move without drying out. Furthermore, they generally remain close to places that could provide an adequate food supply. Without a doubt, the external structure of the earthworm definitely draws a tremendous amount of attention.

External structure of the earthworm, Lumbricus terrestris (Photograph by R.E. Gaddie, 2003) A typical earthworm is about twelve to thirty centimeter long. The earthworm has a bilateral body, which means that it has a distinct head, or anterior end, and a tail, or posterior end. Looking at the image of the external structure of the earthworm, one could see that the earthworm has a mouth at the anterior end. There is an area in front of the mouth called the prostomium that plays a major role in the digesting process of the earthworm. The anus is positioned at the earthworm’s posterior end through which metabolic wastes are excreted to the outside. The earthworm’s body is divided into numerous body segments that are typically covered by setae, pairs of hard, chitinous structures that project from the body wall. The setae’s function is to anchor the worm in its burrow as well as help it craw long the moist burrows. All forms of the earthworm have hydrostatic skeleton that consists of fluid held under pressure in a closed body compartment. The hydrostatic skeleton’s principal role is to enable earthworms to move by peristalsis, a type of locomotion produced by rhythmic waves of muscle contractions passing from head to tail. Earthworms characteristically expand by contracting circular muscles, whereas they could shorten the body by invoking contractions of the longitudinal muscles. Although the earthworms lack eyes, they have many light sensitive organs in some segments. The physiological systems such as circulatory, respiratory, and reproductive systems are also worth of interest and consideration.

The circulatory system in earthworms is closed and complex.  There is a row of muscular blood vessels in earthworms that function as hearts in gas exchange and food transport.  The earthworm does not have an internal respiratory.  In fact, it does not have lungs; it is a skin breather in the sense that it respires through the skin—therefore the earthworm could definitely suffer from dehydration if its skin dries out.  Earthworms are hermaphrodites.  In other words, they possess both male and female organs in the same body.  However, earthworms cross-fertilize simultaneously with the help of a special external structure called a clitellum.  The clitellum secretes mucus that helps hold the earthworms together during mating; it also secretes a mucous cocoon that collects as well as protects the fertilized eggs.  After picking up the fertilized eggs by sliding along the worm’s body, the mucous cocoon would stoop off the worm’s head and deposit into the soil until the eggs hatch.  The digestive system of the earthworm is too extremely interesting.

(Phylum Annelida link, 2003) This is a diagram of the earthworm’s digestive system. Earthworms are deposit feeders in the essence that they would ingest the soil and then extract the nutrients from the soil. Since earthworms have no jaws or teeth, they create a high suction by using its muscular pharynx along with prostomium in order to pull food into its mouth. The food particles and soil are passed along the esophagus into the crop, which is an organ that stores the food temporarily. The food is then ground up into small digestible pieces in the gizzard. After being ground up into small pieces, the nutrients are being absorbed into the body in the small intestine. The indigestible material is then excreted along with feces through the anus to the outside. Earthworms are often preys of birds such as robins and mammals such as badgers and moles. Earthworms are very important to the soil, which certainly has direct effects towards our agriculture. The burrows that are formed by earthworms allow water and air easy entry into the soil; it would let water enter the rooting zone where it can be used by plants. The burrows also allow roots to move easily through the soil into new spaces. However, despite their beneficial contributions to the soil, insecticides that are targeted at other insects sometimes wipe out earthworms.

External references:

Martin, J.P. et al. “Earthworm Biology and Production.” http://edis.ifas.ufl.edu/IN047 (November 2, 2003)

“Phylum Annelida” http://www.esu.edu/~milewski/intro_biol_two/lab__12_annel_arthro/lumbr_internal_anat.html (November 2, 2003)

“Phylum Annelida: Polychaetes, Earthworms, Leeches” http://216.239.53.104/search?q=cache:mgbskuIB4E0J:www.tulane.edu/~bfleury/diversity/diversitylectures/annelids.rtf+Lumbricus+respiration&hl=en&ie=UTF-8 (November 2, 2003)

“The Earthworm” http://web.ukonline.co.uk/webwise/spinneret/pot/odds/worm.htm (November 2, 2003)

University of California. Sustainable Agriculture Research and Education Program. http://www.sarep.ucdavis.edu/worms/image6.htm (November 2, 2003)

Biology 226 Home Page. “Outline for Annelids.” http://faculty.vassar.edu/mehaffey/academic/animalstructure/outlines/annelida.html (November 2, 2003)

Delahut, Haut and Koval. “Earthworms: Beneficials or Pests?” http://www.uwex.edu/ces/wihort/turf/Earthworms.htm (November 2, 2003)