Earthworm
An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class Oligochaeta. In classical systems, they were in the order of Opisthopora since the male pores opened posterior to the female pores, although the internal male segments are anterior to the female. Theoretical cladistic studies have placed them in the suborder Lumbricina of the order Haplotaxida, but this may change. Other slang names for earthworms include "dew-worm", "rainworm", "nightcrawler", and "angleworm". Larger terrestrial earthworms are also called megadriles as opposed to the microdriles in the semiaquatic families Tubificidae, Lumbricidae and Enchytraeidae. The megadriles are characterized by a distinct clitellum and a vascular system with true capillaries.
Earthworms are commonly found in moist, compost-rich soil, eating a wide variety of organic matters, which include detritus, living protozoa, rotifers, nematodes, bacteria, fungi and other microorganisms. An earthworm's digestive system runs the length of its body. They are one of nature's most important detritivores and coprophages, and also serve as food for many low-level consumers within the ecosystems.
Earthworms exhibit an externally segmented tube-within-a-tube body plan with corresponding internal segmentations, and usually have setae on all segments. They have a cosmopolitan distribution wherever soil, water and temperature conditions allow. They have a double transport system made of coelomic fluid that moves within the fluid-filled coelom and a simple, closed circulatory system, and respire via cutaneous respiration. As soft-bodied invertebrates, they lack a true skeleton, but their structure is maintained by fluid-filled coelom chambers that function as a hydrostatic skeleton.
Earthworms have a central nervous system consisting of two ganglia above the mouth, one on either side, connected to an axial nerve running along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors concentrate near its mouth. Circumferential and longitudinal muscles edging each segment let the worm move. Similar sets of muscles line the gut tube, and their actions propel digested food toward the worm's anus.
Earthworms are hermaphrodites: each worm carries male and female reproductive organs and genital pores. When mating, two individual earthworms will exchange sperm and fertilize each other's ova.
Anatomy
Form and function
Depending on the species, an adult earthworm can be from long and wide to long and over wide, but the typical Lumbricus terrestris grows to about long. Probably the longest worm on confirmed records is Amynthas mekongianus that extends up to 3 m in the mud along the banks of the 4,350 km Mekong River in Southeast Asia.From front to back, the basic shape of the earthworm is a cylindrical tube-in-a-tube, divided into a series of segments that compartmentalize the body. Furrows are generally externally visible on the body demarking the segments; dorsal pores and nephridiopores exude a fluid that moistens and protects the worm's surface, allowing it to breathe. Except for the mouth and anal segments, each segment carries bristlelike hairs called lateral setae used to anchor parts of the body during movement; species may have four pairs of setae on each segment or more than eight sometimes forming a complete circle of setae per segment. Special ventral setae are used to anchor mating earthworms by their penetration into the bodies of their mates.
Generally, within a species, the number of segments found is consistent across specimens, and individuals are born with the number of segments they will have throughout their lives. The first body segment features both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals the entrance when the worm is at rest, but is also used to feel and chemically sense the worm's surroundings. Some species of earthworm can even use the prehensile prostomium to grab and drag items such as grasses and leaves into their burrow.
An adult earthworm develops a belt-shaped glandular swelling, called the clitellum, which covers several segments toward the front part of the animal. This is part of the reproductive system and produces egg capsules. The posterior is most commonly cylindrical like the rest of the body, but depending on the species, it may also be quadrangular, octagonal, trapezoidal, or flattened. The last segment is called the periproct; the earthworm's anus, a short vertical slit, is found on this segment.
The exterior of an individual segment is a thin cuticle over the skin, commonly pigmented red to brown, which has specialized cells that secrete mucus over the cuticle to keep the body moist and ease movement through the soil. Under the skin is a layer of nerve tissue, and two layers of muscles—a thin outer layer of circular muscle, and a much thicker inner layer of longitudinal muscle. Interior to the muscle layer is a fluid-filled chamber called a coelom that by its pressurization provides structure to the worm's boneless body. The segments are separated from each other by septa which are perforated transverse walls, allowing the coelomic fluid to pass between segments. A pair of structures called nephrostomes are located at the back of each septum; a nephric tubule leads from each nephrostome through the septum and into the following segment. This tubule then leads to the main body fluid filtering organ, the nephridium or metanephridium, which removes metabolic waste from the coelomic fluid and expels it through pores called nephridiopores on the worm's sides; usually, two nephridia are found in most segments. At the centre of a worm is the digestive tract, which runs straight through from mouth to anus without coiling, and is flanked above and below by blood vessels and the ventral nerve cord, and is surrounded in each segment by a pair of pallial blood vessels that connect the dorsal to the subneural blood vessels.
Many earthworms can eject coelomic fluid through pores in the back in response to stress; the Australian Didymogaster sylvaticus can squirt fluid as high as.
Nervous system
Central nervous system
The CNS consists of a bilobed brain, sub-pharyngeal ganglia, circum-pharyngeal connectives and a ventral nerve cord.Earthworms' brains consist of a pair of pear-shaped cerebral ganglia. These are located in the dorsal side of the alimentary canal in the third segment, in a groove between the buccal cavity and pharynx.
A pair of circum-pharyngeal connectives from the brain encircle the pharynx and then connect with a pair of sub-pharyngeal ganglia located below the pharynx in the fourth segment. This arrangement means the brain, sub-pharyngeal ganglia and the circum-pharyngeal connectives form a nerve ring around the pharynx.
The ventral nerve cord begins at the sub-pharyngeal ganglia and extends below the alimentary canal to the most posterior body segment. The ventral nerve cord has a swelling, or ganglion, in each segment, i.e. a segmental ganglion, which occurs from the fifth to the last segment of the body. There are also three giant axons, one medial giant axon and two lateral giant axons on the mid-dorsal side of the ventral nerve cord. The MGA is 0.07 mm in diameter and transmits in an anterior-posterior direction at a rate of 32.2 m/s. The LGAs are slightly narrower at 0.05 mm in diameter and transmit in a posterior-anterior direction at 12.6 m/s. The two LGAs are connected at regular intervals along the body and are therefore considered one giant axon.
Peripheral nervous system
- Eight to ten nerves arise from the cerebral ganglia to supply the prostomium, buccal chamber and pharynx.
- Three pairs of nerves arise from the subpharyngeal ganglia to supply the second, third and fourth segment.
- Three pairs of nerves extend from each segmental ganglion to supply various structures of the segment.
Movement
On the surface, crawling speed varies both within and among individuals. Earthworms crawl faster primarily by taking longer "strides" and a greater frequency of strides. Larger Lumbricus terrestris worms crawl at a greater absolute speed than smaller worms. They achieve this by taking slightly longer strides but with slightly lower stride frequencies.Touching an earthworm, which causes a "pressure" response as well as a response to the dehydrating quality of the salt on human skin, stimulates the subepidermal nerve plexus which connects to the intermuscular plexus and causes the longitudinal muscles to contract. This causes the writhing movements observed when a human picks up an earthworm. This behaviour is a reflex and does not require the CNS; it occurs even if the nerve cord is removed. Each segment of the earthworm has its own nerve plexus. The plexus of one segment is not connected directly to that of adjacent segments. The nerve cord is required to connect the nervous systems of the segments.
The giant axons carry the fastest signals along the nerve cord. These are emergency signals that initiate reflex escape behaviours. The larger dorsal giant axon conducts signals the fastest, from the rear to the front of the animal. If the rear of the worm is touched, a signal is rapidly sent forwards causing the longitudinal muscles in each segment to contract. This causes the worm to shorten very quickly as an attempt to escape from a predator or other potential threat. The two medial giant axons connect with each other and send signals from the front to the rear. Stimulation of these causes the earthworm to very quickly retreat.
The presence of a nervous system is essential for an animal to be able to experience nociception or pain. However, other physiological capacities are also required such as opioid sensitivity and central modulation of responses by analgesics. Enkephalin and α-endorphin-like substances have been found in earthworms. Injections of naloxone inhibit the escape responses of earthworms. This indicates that opioid substances play a role in sensory modulation, similar to that found in many vertebrates.