Earthworm
Overview
The Earthworm is the common name for invertebrates in the order of Opisthopora or Haplotaxida, which includes hundreds of species, with many different families. As a result, their is great diversity within the term "earthworms". They have other folk names such as nightcrawler, angleworm, dewworm, rainworm, etc.
They have a tube within a tube body type, with segmentation all along their body, each segment called annuli. Each external segment has corresponding internal segmentation and setae/bristles to help with movement. They move by contracting and extending circumferential and longitudinal muscles that are in each segment, as well as using the setae on every segment to get a grip.
Earthworms breathe through their skin, and their whole body is covered with a mucous layer that protects from desiccation (i.e. drying out). They have no eyes, instead using cells on the outside of their body that are light sensitive.
They are hermaphrodites, each earthworm exhibiting both male and female sexual organs.
Earthworms live in temperate and tropical soils all over the world, anywhere there is appropriate water content and temperature for at least part of the year. Though, they are most abundant in forests and grasslands of temperate and tropical regions. Earthworm diet includes any organic matter they encounter, like plant matter, fungi, bacteria, or any other organism small enough to be eaten. Their digestive tract is one way, they eat with their mouth on one end and excrete waste at the other end of their body. The same muscles that allow for their movement through soil also push their food through their digestive system.
Scientific Classification
Kingdom - Animalia
Phylum - Annelida
Class - Clitellata
Order - Opisthopora/Haplotaxida
Reproduction
Earthworms can only produce sexually, not asexually. The popular myth that if one cuts an earthworm in half, it will regenerate into two earthworms is false. While earthworms can regenerate small parts of their bodies, they cannot be split into two. Earthworms have both male and female sexual organs, but still need a partner to reproduce. The clitellum is the slightly discolored segment about one-third of the body length away from the head that produces mucous that aids with mating. The worms line up in opposite directions and exchange sperm, using the excreted mucous as a protectant/pathway. The sperm is then stored in sperm sacs, and later released along with the eggs into cocoons secreted from the mucous of the clitellum. This forms the lemon-shaped cocoons to the right, which can have one or more embryos inside, depending on the species. New earthworms emerge from these cocoons in a few weeks.
Ecology
Earthworms are often classified into a few basic categories, based on morphology, feeding habits, and their microhabitats within the soil. These are called, epigeic, endogeic, and anecic.
Epigeic Earthworms
Epigeic Earthworms tend to be the smallest of the three, less than 10 cm, and the most pigmented. They live mainly on leaf litter or surface soil and eat litter and small microbes. These worms do not make burrows, permanent or temporary.
Endogeic Earthworms
Endogeic Earthworms are moderately sized and unpigmented. They inhabit the mineral level of the soil, ranging from 0-80 cm below ground. Subcategories of endogeic earthworms are: Polyhumic, Mesohumic, Endo-anecic, and Oligohumic. These all inhabit slightly different zones with different organic matter content.
Polyhumic worms live on surface soil or in the root zone and prefer soil with high organic content. They are of a small size, less than 15 cm. They tend to make temporary horizontal burrows.
Mesohumic worms live in soil depths of 0-20 cm and feed on soil from depths of 0-10 cm. These worms make extensive horizontal burrows and are of medium size, 10-20cm.
Endo-Acecic worms live in depths of 0-50 cm, and some make more permanent burrows. They feed on soil from depths of 0-10 cm. They are greater than 20cm in length.
Oligohumic worms live in depths of 15-80 cm and feed on soil from depths of 20-40 cm. They are greater than 20 cm in length and tend to be the largest out of all of these categories.
Anecic Earthworms
These worms live in permanent burrows in the soil. They feed on surface leaf litter that they drag deep into their burrows, as well as soil from any depth they encounter. They are dorsally pigmented (i.e. their back is pigmented) and are greater than 15 cm.
References
[1] "Earthworm" Wikipedia, Retrieved April 27, 2021, from https://en.wikipedia.org/wiki/Earthworm
[2] "Earthworm" National Geographic, Retrieved April 27, 2021, from https://www.nationalgeographic.com/animals/invertebrates/facts/common-earthworm
[3] Edwards, Clive A. "Earthworms" Natural Resources Conservation Service Soils, USDA, Retrieved April 27, 2021, from https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053863
[4] "Earthworm Biology – The Science of the Natural Decomposers" BiologyWise, Retrieved April 27, 2021, from https://biologywise.com/earthworm-biology
[5] Painter, Tammie. "How Do Earthworms Reproduce?" Sciencing.com, Retrieved April 27, 2021, from https://sciencing.com/bring-the-outdoors-inside-with-these-nature-kits-for-kids-13763822.html
[6] Clark, Josh. "How Earthworms Work", HowStuffWorks, Retrieved April 27, 2021, from https://animals.howstuffworks.com/animal-facts/earthworm3.htm
[7] Coleman, David C., Callaham, Mac A., Crossley, D. A. "Fundamentals of Soil Ecology", Third Edition, Retrieved from pages 155-168.