Of all annelids, for our purposes, we are interested in oligochaetes. The body of these animals is a tube segmented outside and inside (metamerized). The surface of the body or cuticle is adorned by small chitinous spines (called quetas), embedded in cuticle glands, which help it move. Entering in the body, below the cuticle, there are circular, longitudinal and oblique muscles whose coordinated work produces the peristaltic movements that are used in the displacements, as if it were a human intestine. The internal cavity is wide and filled with a liquid where phagocytic cells are found, which devour every foreign body, including microorganisms, mainly bacteria. The digestive tract is practically rectilinear. There is a sucking mouth, a powerful pharynx, a shredder bag or gizzard, a food storage maw and a bowel, which flows into an anus, at the end of the body. Limiting up and down the digestive tract, there are two circulatory vessels, and more ventrally a stair-shaped nervous system, but with a pair of nerve ganglia per segment, all of them, connected by longitudinal nerves with a brain located in the first segment ( or first segments) of the body.
They do not have sensory organs per se, but they do have fine function chemoreceptors, although some species may have photoreceptor spots that are stimulated by light changes. In fact, most worms are photophobic and run away from the light, so they have eye spots capable of catching light waves and fleeing from them, burying themselves in the substrate.
Earthworms that interest us are hermaphrodites, that is, they have a male and female reproductive organ, with testicles and ovaries, located in different segments, reportable due to their larger size and that integrate the so-called clitelo. Self-fertilization is not the general norm, although it may occur from time to time. The most frequent is cross-fertilization, in such a way that two individuals join to exchange their reproductive material, after which, the fertilized eggs are evacuated in a bag called cocoon. After embryonic development, small independent worms emerge by direct development.
Most earthworms are capable of carrying out asexual reproduction by regeneration. Indeed, sometimes and given how hostile the substrate where earthworms live, the body can fracture and split into several sections, regenerating each of them the missing part, more effectively the closer the part is to the cephalic zone. In some species, this reproduction is normal, not accidental, and can be done in two ways: first the animal is lengthened excessively and later it is split into the worms that should be left (paratomic regeneration) or vice versa, first it is split and then it regenerates (archithomic regeneration). Whatever it is, it has been used to produce a large population in a short time in captivity.
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