Template:POTD/2016-03-29
All earthworms have bodies composed of repeated segments called metameres separated from one another by circular membranes called septa. The metameres are very similar to one another, but in earthworms there are the following variations (among others): there are no nephridia in the first three segments; there are no nerve ganglia in the first two segments (though several nerves extend there from the third and fourth segments); the prostomium, which is not a complete segment, functions as a flap to close the mouth but is also highly flexible and is used by the worm as a prehensile body part to grasp and pull leaves and grass into its burrow. The large muscular pharynx sucks in the worm’s food which then passes between the animal’s hearts to be stored and moistened in the crop and then ground by fragments of ingested sand and stone in the gizzard before entering the rest of the intestine. The gizzard is considered the animal’s stomach. The nephridia are the only waste-filtering organs—there is no separate digestive gland or liver.Diagram: K.D. Schroeder