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Muscle contractions are the basis of movement in many species, but not all. Some organisms, like sea sponges, branched off from the evolutionary path before muscle cells evolved and are still capable of movement. While all animals have some sort of muscular system, some soft-bodied organisms, like Trichoplax, lack muscle and nerve cells. Other soft-bodied organisms, like flatworms, nematodes, and annelids, have muscles and complete digestive systems. Annelids, in particular, have a closed circulatory system in which blood is pumped along by muscles in blood vessels.
| Characteristics | Values |
|---|---|
| Muscle contractions are the basis of movement | Cheetahs, snails, worms, annelids, sea sponges, Trichoplax |
| Muscle structure | Rigid body parts act as levers/cantilevers to redirect force and produce movement |
| Soft-bodied organisms | Cnidarians, flatworms, roundworms, annelids, molluscs, non-vertebrate chordates, non-panarthropod invertebrates, jelly fungus, lichens, slime molds, Trichoplax |
| Digestive system | Muscles surrounding the tube contract, squeezing food and pushing it along in a process called peristalsis |
| Circulatory system | Annelids have a closed circulatory system in which blood is pumped along by muscles in blood vessels |
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What You'll Learn
Some animals evolved rigid body parts to complement their muscles
All animals have muscles, but not all animals have bones or rigid body parts. Muscle contractions are the basis of movement in many species, but some animals, like sea sponges, branched off from the evolutionary path before muscle cells evolved and are still capable of movement.
Some animals with muscles have soft bodies and lack rigid physical skeletons or frames. These are known as soft-bodied organisms, which roughly correspond to the group Vermes as proposed by Carl von Linné. Soft-bodied organisms include non-panarthropod invertebrates from the kingdom Animalia, as well as some non-vascular plants (mosses and algae), fungi (such as jelly fungus), lichens, and slime molds. While most soft-bodied animals do not have skeletons, some have stiff cuticles (roundworms, water bears) or hydrostatic skeletons (annelids).
Soft-bodied organisms vary enormously in anatomy. Cnidarians and flatworms have a single opening to the gut and a diffuse nerve system. The roundworms, annelids, molluscs, various lophophorate phyla, and non-vertebrate chordates have a tubular gut open at both ends. The lack of hard parts in soft-bodied organisms makes them extremely rare in the fossil record, and their evolutionary histories are often poorly known.
To complement their muscles, some animals evolved rigid body parts upon which the muscles can attach and act as levers or cantilevers to redirect force and produce movement. These rigid parts, known as skeletons, can be internal (as in vertebrates, echinoderms, and sponges) or external (as in arthropods and non-coleoid molluscs). Skeletons also serve as structural elements to resist gravity and ambient pressure, and they can provide protective surfaces that shield internal structures from trauma and exposure to external insults.
The musculoskeletal system refers to the combination of the muscular and skeletal systems in animals, which work together to enable movement. Arthropods, or jointed animals, including insects, spiders, crabs, and lobsters, have exoskeletons, which are external skeletons made of chitin, a complex sugar. In contrast, vertebrates have internal skeletons, and their bones are connected to muscles using dense connective tissue bundled into tendons.
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Sponges can contract without muscles
All animals move, but muscle contractions are the basis of movement in many, but not all species. Some animal groups, such as sea sponges, do not have muscles at all, as they branched off from the evolutionary path before muscle cells evolved. However, sponges are not incapable of movement and can contract without muscles.
Sponges are multicellular organisms consisting of jelly-like mesohyl sandwiched between two thin layers of cells. They usually have tube-like bodies full of pores and channels that allow water to circulate through them. Sponges do not have complex nervous, digestive, or circulatory systems. Instead, they rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove waste.
Sponges have the ability to perform movements that are coordinated all over their bodies, mainly contractions of the pinacocytes, squeezing the water channels and thus expelling excess sediment and other substances that may cause blockages. Some species can contract the osculum independently of the rest of the body. Sponges may also contract in order to reduce the area that is vulnerable to attack by predators.
A group of scientists headed by associate professor Dr. Michael Nickel of Friedrich Schiller University Jena (Germany) is looking into movement without muscles. Using 3D imaging, they were able to show that the inner and outer surfaces, and therefore the epithelial cells (pinacozytes), cause the strong body contractions of sponges. These findings offer new approaches to understanding the evolutionary development of musculature.
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Soft-bodied organisms typically don't have skeletons
While all animals have a muscular system, some evolved rigid body parts for muscles to attach to and act as levers or cantilevers to redirect force and produce movement. These rigid parts also serve as protection from trauma and exposure to external factors like heat, chemicals, and pathogens. However, some soft-bodied animals do have functional skeletons maintained by body fluid hydrostatics, known as hydroskeletons, such as earthworms, jellyfish, and squids.
The lack of hard parts in soft-bodied organisms makes them rare in the fossil record, and their evolutionary histories are often poorly known. They require specific conditions to be preserved, and amber is one rare medium that allows such preservation. Soft-bodied organisms like arthropods, nematodes, and protists are often neglected in palaeoecological reconstructions due to their rarity in the fossil record.
Despite the lack of skeletons, some soft-bodied organisms can grow to very large sizes, especially marine representatives. The heaviest soft-bodied organisms are likely female giant squids, with maximum weights estimated at 275 kilograms (606 lb). Arctic lion's mane jellyfish can also reach comparable sizes.
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Annelids have a closed circulatory system with muscles
Muscle contractions are the basis of movement for many species, although some animal groups do not have muscles as they evolved before muscle cells emerged. These groups, such as sea sponges, are still capable of movement. All animals do have a muscular system of some kind, even if it is not used for movement.
Annelids, a type of segmented worm, are soft-bodied organisms that usually have a closed circulatory system. This means that blood is contained within vessels throughout the body. There are over 17,000 species of annelids, including earthworms and leeches, which can be found in a variety of environments, including saltwater, freshwater, and soil.
The annelid circulatory system includes two major blood vessels that run the length of the worm: the dorsal blood vessel along the top and the ventral blood vessel along the bottom. The ventral vessel transports blood from the head to the tail region, while the dorsal vessel transports blood from the tail to the head. These two vessels are connected within each segment by two smaller blood vessels. There are also many small capillaries that extend into the worm's skin, where gas exchange occurs in most species.
Annelids do not have a well-developed heart. Instead, 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 hearts. The blood of many annelids contains a respiratory pigment dissolved in the plasma, which may be involved in oxygen transport or storage.
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Trichoplax is a tiny animal with no organs, muscles, or nerves
Muscle contractions are the basis of movement in many species. However, some animal groups do not have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. These include sea sponges, which are capable of contracting without muscles.
Trichoplax is a tiny, multicellular animal that is a part of this group. It is only about half a millimeter to three millimeters in size and is usually about 25 μm thick. Trichoplax has no organs, nerves, muscles, blood, or veins. It is a flat sheet of cells, thinner than paper, and is constantly changing shape as it moves, resembling an amoeba.
Despite lacking muscle or nerve cells, Trichoplax can hunt, dissolve, and consume algae with surprising sophistication. It behaves as if it has a nervous system, yet it lacks typical nerves and synapses, or connections between brain cells. Trichoplax uses its cilia, or tiny hairs, on its underside to move by twirling them like propellers. When it finds a patch of algae, it stops and settles down atop it like a suction cup. It then releases chemicals from special cells that break down the algae.
Trichoplax adhaerans, or Trix, is found worldwide, crawling across shallow seafloors and feeding on algae. It is one of four named species in the phylum Placozoa, which are very flat organisms commonly less than 4 mm in diameter, lacking any organs or internal structures. They have two cellular layers: a top epitheloid layer made of ciliated "cover cells" and a bottom layer of cylinder cells with cilia used for locomotion and gland cells without cilia. The lack of a nervous system may place an upper limit on their possible size, as larger motile animals tend to have less coordinated locomotion.
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Frequently asked questions
No, not all organisms have muscles. Some organisms, like sea sponges, branched off from the evolutionary path before muscle cells evolved.
Some organisms without muscles can still move through body contractions. For example, sea sponges can contract without muscles, but the cells responsible for this movement are still unknown.
All animals have a muscular system of some sort. Examples of animals with muscles include cheetahs, snails, worms, and humans.
Trichoplax, also known as Trix, is a tiny multicellular organism without any muscle or nerve cells. Despite lacking a nervous system, it can hunt down and consume algae.
