Athena Vale
Annelida, or annelids, are segmented worms that include earthworms, polychaete worms, and leeches. They are bilateral, coelomate protostomes, meaning they have a body cavity called a coelom that is partitioned by septa (crosswalls). The coelom is filled with fluid, which acts as a hydrostatic skeleton, and is surrounded by both circular and longitudinal muscle fibres. These muscles enable annelids to move by peristalsis (waves of contraction and expansion) or by flexing their bodies while using parapodia (limbs) to crawl or swim. Some annelids also have chitinous hair-like structures called setae, which aid in movement by sticking to the substrate and holding parts of the worm in place.
| Characteristics | Values |
|---|---|
| Muscle type | Circular and longitudinal muscles |
| Muscle function | Movement, contraction, and expansion |
| Muscle layers | Two |
| Muscle location | Body wall, blood vessels, gut, and septa |
| Muscle-powered hairs | Cilia |
| Muscle contractions | Localized due to segmentation |
| Muscle support | Fluid in the coelom |
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What You'll Learn
- Annelids have two types of muscles: circular and longitudinal
- Muscles in the body tube and digestive tube aid in movement
- Annelids have a more complex muscle system than flatworms and nematodes
- Annelids have a closed circulatory system with muscles in blood vessels
- Annelids' longitudinal nerve trunks include giant axons, allowing fast signal transmission
Annelids have two types of muscles: circular and longitudinal
Annelids have a mesoderm with muscle, a central nervous system, and an excretory system. Each of these systems is more complex in annelids than in flatworms or nematodes. Annelids have two types of muscles: circular and longitudinal. These muscles develop from the lining of the coelom (body cavity). Circular muscles make a segment longer and slimmer when they contract, while longitudinal muscles make the segment shorter and fatter when they contract. The septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon".
The parapodia ("limbs") of annelids that have them often bear more complex chetae at their tips. Chetae are made of moderately flexible β-chitin and are formed by follicles, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming microvilli, fine hair-like extensions that increase the area available for secreting the cheta. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers, the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia) and gills.
The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates. Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body), or flex the body while using parapodia to crawl or swim.
Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies.
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Muscles in the body tube and digestive tube aid in movement
Annelids have a mesoderm with muscle, a central nervous system, and an excretory system. The body of an annelid is divided into repeating sections called segments, with many internal organs repeated in each segment. The body wall of annelids has two types of muscles: circular and longitudinal. The circular muscles make a segment longer and slimmer when they contract, while the longitudinal muscles, usually four distinct strips, make the segment shorter and fatter when they contract.
The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. The septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon". The parapodia of annelids that have them often bear more complex chetae at their tips, which are made of moderately flexible β-chitin and are formed by follicles, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta.
Annelids have a closed circulatory system in which blood is pumped along by muscles in blood vessels. The muscles in the body tube and digestive tube can put pressure on the fluid in the coelom to aid in movement. The coelom is a body cavity between the digestive tube and the external body wall that is lined with tissue. The muscles surrounding the digestive tube contract, squeezing the food and pushing it along in a process called peristalsis. Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body), or flex the body while using parapodia to crawl or swim.
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Annelids have a more complex muscle system than flatworms and nematodes
Annelids, like flatworms, have a mesoderm with muscle, a central nervous system, and an excretory system. However, each of these systems is more complex in annelids than in flatworms or nematodes. Annelids have two types of muscles in their body wall: circular and longitudinal. The circular muscles make a segment longer and slimmer when they contract, while the longitudinal muscles, usually consisting of four distinct strips, make the segment shorter and fatter when they contract. The septa enable the circular and longitudinal muscles to change the shape of individual segments, making each segment a separate fluid-filled "balloon".
In contrast, flatworms, the simplest worms, have a less complex musculature consisting of three layers: a layer of circular muscle fibres immediately under the epidermis, a layer of diagonal fibres, and a deeper longitudinal layer. Flatworms also have dorsoventral muscle fibres running from the upper to the lower epidermis of their flattened bodies. These muscles enable flatworms to crawl in a snail-like fashion by passing waves of muscular contraction along their bodies.
Nematodes also have longitudinal muscles, but they lack circular muscles. Nematodes move by contracting their longitudinal muscles, thrashing and wriggling rather than crawling. The muscle cells of nematodes are unusual in that they send branches to the neurones, unlike in other animals where neurones send processes to the muscle cells.
In addition to their more complex musculature, annelids have evolved body features not found in flatworms or nematodes. For example, the parapodia ("limbs") of annelids often bear more complex chetae at their tips, such as jointed, comb-like, or hooked structures. These structures are made of moderately flexible β-chitin and are formed by follicles, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta.
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Annelids have a closed circulatory system with muscles in blood vessels
Annelids are invertebrates that possess a mesoderm with muscle, a central nervous system, and an excretory system. They have a closed circulatory system, which means that blood is always contained within blood vessels. This is in contrast to open circulatory systems, where blood (or hemolymph) may constitute a much higher percentage of body weight and is less distinct from interstitial fluids.
The annelid circulatory system includes two major blood vessels that run the length of the worm: the dorsal blood vessel along the top and a ventral blood vessel along the bottom. The ventral vessel transports oxygenated blood from the head to the tail region, while the dorsal vessel transports deoxygenated blood from the tail to the head. These two vessels are connected to each other within each segment by two smaller blood vessels. There are also many small capillaries that extend into the skin of the worm, and this is where gas exchange occurs in most species.
Annelids do not have a central, well-developed heart, and usually, the muscular dorsal blood vessel functions to pump blood through the circulatory system. In some species, there may be several muscular blood vessels that function as blood-pumping hearts. The circulatory system of annelids is more complex than that of flatworms or nematodes.
The body wall of annelids contains two types of muscles: circular and longitudinal. These muscles enable annelids to change the shape of individual segments, making each segment a separate fluid-filled "balloon". Circular muscles make a segment longer and slimmer when they contract, while longitudinal muscles make the segment shorter and fatter.
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Annelids' longitudinal nerve trunks include giant axons, allowing fast signal transmission
Annelids, or segmented worms, include species such as earthworms and leeches. They have a mesoderm with muscle, a central nervous system, and an excretory system. The body wall of annelids contains two types of muscles: circular and longitudinal. The longitudinal muscles, usually consisting of four distinct strips, contract to make the segment shorter and fatter.
Annelids' longitudinal nerve trunks include giant axons, which are the output signal lines of nerve cells. These giant axons have a large diameter, which decreases their resistance and allows them to transmit signals very quickly. This rapid transmission enables annelids to contract their bodies very rapidly, which is essential for escaping predators.
The giant axons in annelids can be simple or compound neurochords. Simple neurochords are large single nerve cells, while compound neurochords are multiple structures with each axon connected to multiple cell bodies. The giant nerve cord facilitates the rapid transmission of impulses from one end of the worm to the other, allowing the longitudinal muscles of each segment to contract simultaneously.
The large size of these giant axons enables the rapid conduction of impulses to the segmental muscles. This rapid conduction results in extremely rapid movement, which is crucial for the worm's survival when reacting to potential dangers.
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Frequently asked questions
Yes, annelids have muscles.
Annelids have two types of muscles: circular and longitudinal.
When the circular muscles contract, the segment becomes thinner and longer.
Contractions of the longitudinal muscles make the segment shorter and fatter.
