Athena Vale
All animals have a muscular system of some sort, but not all animals have rigid physical skeletons or frames. Invertebrates such as slugs, worms, and jellyfish have no skeleton, and their movement is not produced by lever action. Arthropoda is the largest phylum of invertebrate animals and includes crustaceans, insects, arachnids, and other classes. Arthropods have striated muscles, similar to the skeletal muscles of humans, which enable the development of flight in many insects. Other phyla with muscles include Cnidaria, Mollusca, Platyhelminthes (flatworms), and Nematoda (roundworms).
| Characteristics | Values |
|---|---|
| Phylum with muscles | Cnidaria, Platyhelminthes, Nematoda, Mollusca, Arthropoda |
| Types of muscles | Longitudinal, circular, diagonal, dorsoventral, striated, obliquely striated, smooth |
| Muscles in soft-bodied organisms | Most soft-bodied organisms have muscles, but no skeleton. Some have hydroskeletons. |
| Muscles in arthropods | Striated, attached to inner surface of exoskeleton |
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What You'll Learn
Phylum Arthropoda
Arthropods are invertebrates with a non-living exoskeleton composed of chitin, a complex sugar, bound to protein. They have a body with differentiated segments, and paired jointed appendages. They must go through moulting to continue growing, a process by which they shed their exoskeleton to reveal a new one. They have an open circulatory system, with a body cavity called a haemocoel through which haemolymph circulates to the interior organs. Arthropods have ladder-like nervous systems, with paired ventral nerve cords running through all segments and forming paired ganglia in each segment.
The four subphyla of extant forms in Phylum Arthropoda are Chelicerata (arachnids), Crustacea (crustaceans), Hexapoda (insects and springtails), and Myriapoda (millipedes and centipedes). The extinct subphylum Trilobitomorpha includes trilobites, the dominant arthropods in the early Paleozoic era.
Arthropods are represented in every habitat on Earth and show a great variety of adaptations. They are important members of marine, freshwater, land, and air ecosystems. They are one of only two major animal groups that have adapted to life in dry environments, the other being amniotes, which include reptiles, birds, and mammals.
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Phylum Mollusca
The molluscs include many familiar animals, including clams, snails, slugs, and squid, as well as some lesser-known animals, like tusk shells and chitons. They range in size from the 20-meter-long giant squid to microscopic aplacophorans, which are a millimeter long. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes. About 80% of all known mollusc species are gastropods.
There are four major groups within the phylum Mollusca: Class Polyplacophora consists of chitons, snail-like molluscs with eight-part overlapping scale shells. Class Gastropoda are true snails and slugs, representing the most diverse class within phylum Mollusca with 60,000 to 80,000 extant species in marine, freshwater, and terrestrial habitats. Class Bivalvia are molluscs with hinged two-part shells, including clams, oysters, mussels, and scallops. Class Cephalopoda are molluscs with large heads, large eyes, and grasping tentacles, including octopuses, squid, cuttlefish, and nautiloids.
Molluscs have a mantle or mass of soft flesh that covers the soft body and encloses the internal organs. In many species, the mantle produces a hard shell. The mouth structures of many molluscs include a specially adapted rasp-like tongue called a radula, which is used for feeding. The radula is a hard ribbon-shaped structure covered in rows of teeth. The radula of a snail is used like a file, rasping back and forth to scrape off small bits of food.
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Phylum Cnidaria
Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found in both freshwater and marine environments. Cnidarians are diploblastic animals, meaning they have two main cell layers, while more complex animals have three. The two layers of cells in cnidarians form epithelia that are mostly one cell thick and are attached to a fibrous basement membrane. They also secrete the jelly-like mesoglea that separates the layers. The outer layer, the ectoderm, has cells that aid in capturing food and cells that secrete mucus. The layer facing outwards, known as the ectoderm, contains epithiliomuscular cells whose bodies form part of the epithelium but whose bases extend to form muscle fibres in parallel rows. The inward-facing layer, the endoderm, has cells that produce digestive enzymes and break up food particles.
Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four main classes: Anthozoa (sea anemones, corals, sea pens); Scyphozoa (jellyfish and moon jellies); Cubozoa (box jellies); and Hydrozoa (a diverse group that includes all the freshwater cnidarians as well as many marine forms). Cnidarians have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles. Many cnidarians take both forms during their life cycles. The polyp form has a body shaped like a hollow cylinder or a bag that opens and closes at the top, with tentacles forming a ring around a small mouth at the top of the bag. The mouth leads to a central body cavity, the gastrovascular cavity. The body of a medusa, commonly called a jellyfish, usually has the shape of a bell or an umbrella, with tentacles hanging downward at the margin.
Cnidarians are the most primitive of animals whose cells are organized into distinct tissues, but they lack organs. They are distinguished by an uncentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the few animals that can reproduce both sexually and asexually.
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Phylum Platyhelminthes
The word "phylum" is derived from the Greek word "phylon", which means "race" or "stock". It is a taxonomic rank used in biology to classify organisms based on their evolutionary relatedness or morphological similarity. Phyla are typically defined by a set of characteristics shared by all members of that phylum.
Platyhelminthes, or flatworms, is a phylum of simple bilaterian, unsegmented, soft-bodied invertebrates. They are acoelomates, meaning they lack an internal body cavity, circulatory system, and respiratory organs. Flatworms are typically small, with free-living non-parasitic flatworms usually less than 10 centimeters long. They live on land, in fresh water, in the ocean, and sometimes on or inside other organisms as parasites. Flatworms have a distinctive flattened shape, which allows oxygen and nutrients to pass through their bodies via diffusion.
Platyhelminthes are divided into two subgroups: the Turbellaria, which are mostly non-parasitic, and three parasitic groups, Cestoda, Trematoda, and Monogenea. However, this classification is now considered outdated as the Turbellaria are not monophyletic, meaning they do not include all descendants of a common ancestor.
Flatworms have a simple body structure, with three main cell layers (endoderm, mesoderm, and ectoderm). While they do not have muscle cells in the same way that vertebrates do, some flatworms have been found to have musculoepithelial cells, which serve a dual function as muscle and epithelial cells. Flatworms have a layer of circular muscle fibres under the epidermis, followed by a layer of diagonal fibres, and a longitudinal layer. These muscle fibres allow the flatworm's body to change shape and length and enable the worm to crawl by passing waves of muscular contraction along its body.
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Phylum Nematoda
The phylum Nematoda, also known as the phylum of roundworms, contains about 25,000 species of nematodes that have been formally described by scientists. Nematodes are found in almost every habitat on Earth. They are long and narrow, resembling tiny threads, which is where they get their name—from the Greek word "nema", meaning thread.
The epidermis (skin) of a nematode is unusual; it is not composed of cells like other animals, but instead is a mass of cellular material and nuclei without separate membranes. This epidermis secretes a thick outer cuticle, which is both tough and flexible. The cuticle is periodically shed during the life of a nematode as it grows, usually four times before it reaches adulthood. The cuticle acts as a support structure and leverage point for movement.
Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles. These muscles are activated by two nerves that run the length of the nematode on both the dorsal (back) and ventral (belly) sides. The nerves have a series of nerve centres along their length, and both nerves connect to a nerve ring and additional nerve centres located near the head. The muscles lie just underneath the epidermis and are responsible for the characteristic thrashing movement of nematodes. The contraction of these longitudinal muscles causes the body to flex, resulting in the worm moving by thrashing back and forth.
Nematodes have a simple body plan and nervous system. The nervous system consists of a ring of nervous tissue around the pharynx, which gives rise to dorsal and ventral nerve cords running the length of the body. The head of a nematode has a few tiny sense organs, and a mouth opening into a muscular pharynx (throat) where food is pulled in and crushed. This leads into a long simple gut cavity lacking any muscles, and then to an anus near the tip of the body.
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Frequently asked questions
All animals have a muscular system of some sort. Arthropoda is the largest phylum of invertebrate animals and comprises crustaceans, insects, arachnids, and other classes. The muscles for movement are attached to the inner surface of the exoskeleton. Another phylum with muscles is Mollusca, which includes snails, slugs, bivalves, and cephalopods.
Some examples of invertebrates with muscles include worms, slugs, jellyfish, sea anemones, and hydras.
The types of muscles in the Arthropoda phylum include dorsoventral muscles, which run vertically from the sternum to the tergum, and longitudinal muscles, which run lengthwise along the segment.
