Victor Forge
Animals have different types of musculoskeletal systems, which are made up of the muscular and skeletal systems. Muscles are tissues specialized in contraction, and they are present in almost all metazoans. Arthropods, such as insects, spiders, and crustaceans, have exoskeletons, which are external skeletons made of chitin, a type of complex sugar. Invertebrates like slugs and worms have no skeleton and rely on their muscles for movement. Even vertebrates have muscular parts that are not connected to the skeleton, such as the tongue. Some of the simplest multicellular animals with muscles include hydras, which are small, freshwater creatures with a mouth surrounded by tentacles.
| Characteristics | Values |
|---|---|
| Animals with muscles | Slugs, worms, hydras, jellyfishes, sea anemones, spiders, crabs, lobsters, shrimps, octopuses, squids, vertebrates, mammals |
| Muscles in animals with no skeleton | Movement in these animals is not produced by lever action |
| Muscles in animals with a skeleton | Muscles are attached to bones to enable movement |
| Types of muscles | Longitudinal, circular, radial, transverse |
| Muscle structure | Muscles have fibres that can be longitudinal, circular, or both |
| Muscle function | Muscles contract and relax to enable movement; they also have regenerative capacities |
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What You'll Learn
- Mollusks, including abalone Haliotis, have muscles that connect their shells to their feet
- Worms, slugs, and other invertebrates have no skeleton, so they move using muscles
- Arthropods, including insects, spiders, and crustaceans, have exoskeletons and muscles
- Humans have over 650 skeletal muscles, with most muscles working in pairs
- Some animals, like sponges, lack true muscles but can still contract their bodies
Mollusks, including abalone Haliotis, have muscles that connect their shells to their feet
All animals have muscles, and they work together with the skeletal system to enable movement. The musculoskeletal system is a combination of the muscular and skeletal systems. While the skeleton provides support for the body, the muscles attached to the bones help the body move.
Molluscs, including abalone Haliotis, are a class of soft-bodied gastropods (snails, slugs, and abalone) that have muscles connecting their shells to their feet. The Haliotid family, which includes abalone, has a worldwide distribution along the coastal waters of every continent, except for the Pacific coast of South America, the Atlantic coast of North America, the Arctic, and Antarctica. The shell of the abalone is exceptionally strong and is made of microscopic calcium carbonate tiles stacked like bricks. The soft body of the abalone is coiled around the columellar muscle, and its insertion is on the middle of the inner wall of the shell, instead of being on the columella. The abalone's foot is very large and rounded, and it uses this broad, muscular foot to cling to rocky surfaces at sublittoral depths.
The shell muscle of the abalone Haliotis connects the domed shell of the animal to its adhesive foot. When the muscle shortens, with the foot attached to a rock, the shell is pulled down over the animal to protect it. Conversely, when the muscle lengthens due to the contraction of circular and radial fibres, the shell is raised from the rock, allowing respiratory water currents to circulate.
Molluscs are highly diverse, not just in size and anatomical structure but also in behaviour and habitat. They are the largest marine phylum, comprising about 23% of all named marine organisms. They are also the second-largest animal phylum, with around 76,000 extant species recognised.
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Worms, slugs, and other invertebrates have no skeleton, so they move using muscles
Animals with muscles include mollusks, hydras, jellyfishes, sea anemones, worms, slugs, and other invertebrates. Unlike vertebrates, which move by applying leverage to their bones with muscles, invertebrates like worms and slugs have no skeleton. Instead, they move using muscles oriented in certain ways along with body fluids. This is known as a hydrostat or hydroskeleton, similar to a water balloon, where squeezing one end causes the water to push out and the balloon to lengthen in the opposite direction.
Invertebrates with muscles include the worm, the elephant's trunk, and the squid. The squid, for example, has helical muscles in its tentacles that act like springs. When these muscles contract, they can shoot out and retract its tentacles with great speed and force. This is also seen in the shell muscle of the abalone Haliotis, which connects the animal's domed shell to its adhesive foot. When the muscle shortens, the shell is pulled down to protect the animal, and when the muscle lengthens, the shell is raised, allowing for respiratory water currents to circulate.
Worms, such as earthworms, are another example of invertebrates that move using muscles without a skeleton. They have longitudinal and circular muscles that work together to create movement. By contracting and loosening these muscles, worms can move forward without rocking side-to-side. This is also seen in inchworms, which have a different type of muscle arrangement that allows them to move in a snake-like "S" curve.
Slugs, as shell-less terrestrial gastropod molluscs, also fall into this category of invertebrates with muscles but no skeleton. While they may not have the same muscle structure as worms, they do possess muscles that enable them to move and navigate their environment. Slugs also produce two types of mucus, one thin and watery, and the other thick and sticky, which help them navigate vertical surfaces and leave a "slime trail" that can be used for finding mates or hunting prey.
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Arthropods, including insects, spiders, and crustaceans, have exoskeletons and muscles
Arthropods are invertebrates with segmented bodies and jointed limbs. They include insects, spiders, and crustaceans such as crabs and lobsters. These animals have an exoskeleton, which is a hardened shell or exterior skeleton that covers their interior parts. The exoskeleton is made of chitin, a type of complex sugar or polymer of N-Acetylglucosamine. It consists of two layers: a thin, outer protein layer called the epicuticle, and a thick, inner chitin-protein layer called the procuticle. The exoskeleton provides support, protection, and contributes to efficient locomotion, with the muscle system attached to its inner surface.
The process of growth in arthropods involves moulting, or shedding their old exoskeleton and forming a larger one to allow for expansion. This process is regulated by hormones and environmental factors, and the interval between moults is called an instar. Some arthropods, like spiders and insects, stop moulting upon reaching sexual maturity, while others, like lobsters and crabs, moult throughout their lives.
The smallest arthropods belong to the class Tantulocarida, measuring less than 100 micrometres in length. In contrast, the largest arthropods are species in the class Malacostraca, with the Japanese spider crab's legs spanning up to 4 metres, and the American lobster weighing over 20 kg. Arthropods have a wide range of appendages for various functions, including eating, sensing, mating, walking, and defence.
The success of arthropods can be attributed to their unique, jointed exoskeleton, which provides both support and protection. Additionally, their muscle system, attached to the inside of the exoskeleton, enables efficient locomotion. Some arthropods, like spiders, also use hydraulic pressure to extend their legs, generating pressures up to eight times their resting level.
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Humans have over 650 skeletal muscles, with most muscles working in pairs
Animals have different types of musculoskeletal systems. Arthropods, or jointed animals, include insects, spiders, and crustaceans such as crabs and lobsters. These animals have an exoskeleton, a hardened shell or exterior skeleton that covers their interior parts.
In contrast, humans have an internal skeleton with over 650 skeletal muscles, making up around 30% to 40% of our total body mass. These muscles are under our voluntary control and allow us to perform a wide range of movements and functions. Most of our muscles work in pairs, with one muscle contracting while the other relaxes, enabling fluid and flexible movements.
Skeletal muscles are composed of flexible muscle fibers that range from less than half an inch to just over 3 inches in diameter. These fibers can contract and tighten, allowing the muscles to move the bones and enabling various movements. Each muscle can contain hundreds to thousands of these fibers, which are surrounded by different layers of connective tissue known as the epimysium, perimysium, and endomysium.
The human body has three types of muscles: skeletal, cardiac, and smooth muscle. Skeletal muscles are the most common and are attached to bones by tendons, allowing for movement. Cardiac and smooth muscles are involuntary and controlled by the autonomic nervous system, performing essential functions without conscious thought.
Understanding animal musculoskeletal systems provides insights into evolution, movement, and species classification. The study of animal skeletons, including extinct species like dinosaurs, helps scientists trace the evolution of birds from dinosaurs and classify different animal groups.
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Some animals, like sponges, lack true muscles but can still contract their bodies
Muscle contractions are the basis of movement in many species. However, some animals, like sponges, can contract their bodies without muscles. This is because they branched off from the evolutionary path before muscle cells evolved.
Sponges possess epithelial cells, which are believed to be evolutionary predecessors of muscle cells. Scientists have found that sponge epithelial cells and the muscle cells of other animals share a common contractile cellular predecessor. This discovery has provided new insights into the evolutionary development of musculature.
Hydras are another example of simple multicellular animals that possess muscle. They are hollow, cylindrical, freshwater creatures about 10 mm long. One end attaches to a plant or another support, while the other end has a mouth surrounded by tentacles. The body wall consists of two layers of cells with a middle gelatinous layer called mesoglea. In hydras, one kind of cell serves as both muscle and epithelial cell. The compact body of each cell is packed closely with adjacent cells to form an epithelium, and the base of each cell is drawn out into a long muscle fibre.
Other soft-bodied animals, such as slugs and worms, lack a skeleton and produce movement without lever action. They have muscle systems based on the principle of longitudinal and circular muscle fibres working antagonistically. If the longitudinal muscles contract and the body shortens, it must widen to maintain its volume. If the circular muscles contract and the body thins, it must lengthen.
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Frequently asked questions
All animals have muscles, as they are needed for movement.
Invertebrates with no skeleton, such as slugs, worms, and mollusks, have muscles. Hydras, which are simple multicellular freshwater animals, also have muscles.
The skeleton provides support for the body and allows for movement with the help of muscles attached to bones. The brain sends a signal through the nervous system to trigger the muscles.
