Acoelomate Definition and Examples

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Acoelomate Definition and Examples

Body Plans - Acoelomate
Triploblasts may be acoelomates, eucoelomates, or pseudocoelomates. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm tissue. Pseudocoelomates have a similar body cavity, but it is lined with mesoderm and endoderm tissue. OpenStax, Features of the Animal Kingdom/CC BY 3.0

An acoelomate is defined as an animal that does not possess a body cavity. Unlike coelomates (eucoelomates), animals with a true body cavity, acoelomates lack a fluid-filled cavity between the body wall and digestive tract. Acoelomates have a triplobastic body plan, meaning that their tissues and organs develop from three primary embryonic cell (germ cell) layers. These tissue layers are the endoderm (endo-, -derm) or innermost layer, mesoderm (meso-, -derm) or middle layer, and the ectoderm (ecto-, -derm) or outer layer. Different tissues and organs develop in these three layers. In humans, for example, the epithelial lining that covers internal organs and body cavities is derived from the endoderm. Muscle tissue and connective tissues such as bone, blood, blood vessels, and lymphatic tissue are formed from mesoderm. Urinary and genital organs including the kidneys and gonads are also formed from mesoderm. Epidermis, nervous tissue, and specialized sense organs (eyes, ears, ect.) develop from the ectoderm.

Coelomates have a body cavity within the mesoderm that is fully lined by mesoderm tissue. Acoelomates have a middle layer that has no cavity and is completely filled by mesoderm tissue and organs. Pseudocoelomates have a body cavity, however the cavity is not fully lined by mesoderm tissue. The lack of a coelom means that acoelomate organs are not as well protected against external pressure and shock as are the organs in coelomates.

Acoelomate Characteristics

In addition to not having a body cavity, acoelomates have simple forms and lack highly developed organ systems. For example, acoelomates lack a cardiovascular system and respiratory system and must rely on diffusion across their flat, thin bodies for gas exchange. Acoelomates commonly possess a simple digestive tract, nervous system, and excretory system. They have sense organs for detecting light and food sources, as well as specialized cells and tubules for eliminating waste. Acoelomates commonly have a single orifice that serves as both an inlet for food and an exit point for undigested waste. They have a defined head region and display bilateral symmetry (can be divided into two equal left and right halves).

Acoelomate Examples

Examples of acoelomates are found in the kingdom Animalia and the phylum Platyhelminthes. Commonly known as flatworms, these invertebrate animals are unsegmented worms with bilateral symmetry. Some flatworms are free-living and commonly found in freshwater habitats. Others are parasitic and often pathogenic organisms that live within other animal organisms. Examples of flatworms include planarians, flukes, and tapeworms. Ribbon worms of the phylum Nemertea have historically been considered to be acoelomates. However, these mainly free-living worms have a specialized cavity called a rhynchocoel that some consider to be a true coelom.

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Flatworm Planarian
Flatworm Dugesia subtentaculata. Asexual specimen from Santa Fe, Montseny, Catalonia. Eduard Solà /Wikimedia Commons/CC BY 3.0

Planarians are free-living flatworms from the class Turbellaria. These flatworms are commonly found in freshwater habitats and in moist soil environments. They have elongated bodies and most species are brown, black, or white in color. Planarians have cilia on the underside of their bodies which they use for movement. Larger planarians may also move as a result of muscular contractions. Notable characteristics of these flatworms are their flat bodies and triangular shaped heads with a clump of light-sensitive cells on each side of the head. These eye spots function to detect light and also make the worms look as if they are cross-eyed. Special sensory cells called chemoreceptor cells are found in the epidermis of these worms. Chemoreceptors respond to chemical signals in the environment and are used to locate food.

Planarians are predators and scavengers that commonly feed on protozoans and small worms. They feed by projecting their pharynx out of their mouths and onto their prey. Enzymes are secreted that help to initially digest the prey before it is sucked into the digestive tract for further digestion. Since planarians have a single opening, any undigested material is expelled through the mouth.

Planarians are capable of both sexual and asexual reproduction. They are hermaphrodites and have both male and female reproductive organs (testes and ovaries). Sexual reproduction is most common and happens as two planarians mate, fertilizing eggs in both flatworms. Planarians may also reproduce asexually through fragmentation. In this type of reproduction, the planarian divides into two or more fragments that can each develop into another fully formed individual. Each of these individuals are genetically identical.

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Schistosomes Parsitic Worms
Colored scanning electron micrograph (SEM) of adult female (pink) and male (blue) Schistosoma mansoni parasitic worms, cause of the disease bilharzia (schistosomiasis). These parasites live in the veins of the intestines and bladder of humans. Females live in a groove on the males backs. They feed on blood cells, attaching themselves to the vessel walls by a pad on their heads (males at upper right). Females lay eggs continuously, which are excreted in feces and urine. They develop in water snails into forms which infect humans through contact. NIBSC/Science Photo Library/Getty Images

Flukes or trematodes are parasitic flatworms from the class Trematoda. They may be internal or external parasites of vertebrates including, fish, crustaceans, mollusks, and humans. Flukes have flat bodies with suckers and spines that they use to attach to and feed off of their host. Like other flatworms, they have no body cavity, circulatory system, or respiratory system. They have a simple digestive system consisting of a mouth and digestive pouch.

Some adult flukes are hermaphrodites and have both male and female sex organs. Other species have distinct male and female organisms. Flukes are capable of both asexual and sexual reproduction. They have a life cycle that typically includes more than one host. The primary stages of development occur in mollusks, while the latter mature stage occurs in vertebrates. Asexual reproduction in flukes most often occurs in the primary host, while sexual reproduction most often occurs in the final host organism.

Humans are sometimes the final host for some flukes. These flatworms feed off of human organs and blood. Different species may attack the liver, intestines, or lungs. Flukes of the genus Schistosoma are known as blood flukes and cause the disease schistosomiasis. This type of infection causes fever, chills, muscle aches, and if left untreated, may result in an enlarged liver, bladder cancer, spinal cord inflammation, and seizures. Fluke larvae first infect snails and reproduce within them. The larvae leave the snail and infest water. When the fluke larvae come in contact with human skin, they penetrate the skin and enter the bloodstream. The flukes develop within veins, feeding off of blood cells until reaching adulthood. When sexually mature, males and females find one another and the female actually lives within a channel on the males back. The female lays thousands of eggs that ultimately leave the body through the host's feces or urine. Some eggs may become trapped in body tissues or organs causing inflammation.

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Tapeworm, Taenia
Colored scanning electron micrograph (SEM) of a parasitic tapeworm (Taenia sp.). The scolex (head, at right) has suckers (upper right) and a crown of hooklets (top right) that the worm uses to attach itself to the inside of the intestines of its specific host. At the end of the scolex is a narrow neck from which body segments (proglottids) are budded off. Tapeworms have no specialized digestive system but feed on the half- digested food in the intestines by direct absorption through their entire skin surface. Power and Syred/Science Photo Library/Getty Images

Tapeworms are long flatworms of the class Cestoda. These parasitic flatworms can grow in length from less than 1/2 inch to over 50 feet. They may inhabit one host in their life cycle or may reside in intermediate hosts before maturing in a final host. Tapeworms live in the digestive tract of several vertebrate organisms including fish, dogs, pigs, cattle, and humans. Like flukes and planarians, tapeworms are hermaphrodites. However, they are capable of self-fertilization.

The head region of the tapeworm is called the solex and it contains hooks and suckers for attaching to a host. The elongated body contains several segments called proglottids. As the tapeworm grows, the proglottids furtherest away from the head region detach from the tapeworm body. These structures contain eggs that are released into the host's feces. A tapeworm does not have a digestive tract, but obtains nourishment through the digestive processes of its host. Nutrients are absorbed through the outer covering of the tapeworm's body.

Tapeworms are spread to humans by the ingestion of undercooked meat or substances contaminated with egg infested fecal matter. When animals such as pigs, cattle, or fish, ingest tapeworm eggs, the eggs develop into larvae in the animal's digestive tract. Some tapeworm larvae can penetrate the digestive wall to enter a blood vessel and be carried by blood circulation to muscle tissue. These tapeworms become enveloped in protective cysts that remain lodged in the tissue of the animal. Should the raw meat of an animal infested with tapeworm cysts be eaten by a human, adult tapeworms will develop in the digestive tract of the human host. The mature adult tapeworm sheds segments of its body (proglottids) containing hundreds of eggs in the feces of its host. The cycle will begin anew should an animal consume feces contaminated with tapeworm eggs.


  • "Features of the Animal Kingdom." OpenStax CNX. Apr 8, 2013
  • "planarian." The Columbia Encyclopedia, 6th ed.. . April 23, 2017.
  • "Parasites - Schistosomiasis." Centers for Disease Control and Prevention. Last Reviewed November 7, 2012.