The terms, coelomate and acoelomate, refers to the presence or absence of a body cavity,i.e., coelom in animals. The coelom is a fluid-filled structure between the body wall and internal organs. This structure allows for better organ development and movement. The coelom plays a key role in protection of the internal organs and allows complex body functions.
Animals having true coelom are called coelomates, while those who do not have a body cavity are called acoelomates. Knowing this difference will help in classifying animals and studying their body organization. Coelomate and acoelomate are important topics of the subject biology.
A coelom is a fluid-filled cavity found between the body wall and internal organs in animals, and is completely lined by mesoderm. The mesoderm are a layer of cells formed during embryonic development. The cavity gives enough space for development, growth, and movement of internal organs and helps in protection of internal organs from mechanical shocks. The presence or absence of coelom is one of the key features in classifying animals into groups like coelomates and acoelomates.
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A coelomate is any animal with a true coelom, which is a fluid-filled body cavity lined with tissue called mesothelium. The lining of the coelom is completely on all sides, and it surrounds the organs that are suspended within. This cavity provides cushioning and protection for the organs and gives them room to move independently from the outer body wall.
Presence of true coelom lined by mesothelium.
It is within the coelomic cavity that suspension of the organs takes place.
This allows for development and specialisation of organs.
It is found in very different groups of animals annelids, molluscs, arthropods, echinoderms, and chordates.
Allows for a fast flow of body fluids and provides for a hydrostatic skeleton to be flexible.
An acoelomate, on the other hand, lacks a true coelom. These types of organisms have a solid body structure without any fluid-filled cavity existing between the gut and the body wall. The organs are packed within the body space and are not suspended in a separate cavity. The absence of a coelom reduces the body plan to simplicity and narrows down the complexity of organ systems as compared to coelomates.
Absence of a true coelom, the body cavity is instead filled with mesenchyme, a type of loose connective tissue.
Organs lie directly in contact with the body wall.
Found in more primitive animals like flatworms, and Platyhelminthes.
Less room for organ specialisation and movement.
Only relies on the process of diffusion for the distribution of nutrients and waste products.
Coelomates are animals with a fluid-filled body cavity lined with mesoderm, and also allows organs to be protected and move freely. Acoelomates do not have this body cavity, and the internal organs are present directly in the solid tissue. The difference lies between their body structure, movement, and complexity of the organ system. The table below summarises the major anatomical differences between Coelomates and Acoelomates. Explore more Differences and Comparisons Articles in Biology to deepen your knowledge of key concepts.
Feature | Coelomates | Acoelomates |
Body cavity | True coelom with mesothelium lining | Absence of true coelom and mesenchyme filled body cavity |
Embryonic development | Mesoderm formed by gastrulation, and then coelomic cavity develops | Simple development and lack of coelom formation |
Organ arrangement | Organ suspended in coelomic cavity | Organs present in solid body tissue |
Complexity of organs | Complex and specialised | Less complex and directly in contact with the body |
Movement and Flexibility | Increased mobility because of fluid-filled cavity | Limited movement and relies on body wall for support |
Examples | Flatworms (Platyhelminthes) |
In the process of gastrulation, a blastula is changed into a gastrula through cell invagination. After that, there is the formation of ectoderm, mesoderm, and endoderm. Coelomates develop their coelomic cavities by the process of schizocoely in protostomes or by enterocoely in deuterostomes. Fluid-filled spaces are formed in these ways that will house organs and allow for specialisation later on.
In the case of acoelomates, there is no true coelom, rather, organs are directly packed in a solid body structure filled with mesenchyme. The simpler arrangement differing from that of the coelomates reflects the evolutionary adaptations of streamlined body plans toward simpler ecological niches.
Physiological adaptations linked with coelomates and acoelomates are significantly different. In most cases, the digestive system of coelomates is normally more complex, having specialised regions for digestion and absorption, which is carried out by these fluid-filled coelomic cavities. In acoelomates, the structure is simpler and always has one opening through which food is ingested, and afterwards, the waste products are expelled, and have solid body structure and direct organ layout.
Circulation in the body may either be through an open or a closed system in coelomates. In higher and more advanced coelomates, it has a closed system with blood flowing through vessels for improved flow, hence efficient transport of nutrients and removal of waste products. Acoelomates, having no specialised circulatory system, distribute nutrients and remove waste products through diffusion across their body wall.
The coelomates do have a fluid-filled coelom that provides them with a hydrostatic skeleton, improving flexibility and mobility, hence allowing a wide range of locomotory capabilities, from burrowing to swimming. The presence of this internal support system will permit the evolution of highly specialised organs or complex respiratory tract and digestive systems and sophisticated sensory organs.
While acoelomates don't have a coelom, and in its place, there is direct contact between organs and the body wall. Because of this overall simpler structure, it limits their locomotion to crawling or gliding and usually less specialised organ systems.
The development of body cavities, particularly the coelomic cavities in coelomates, is a strong adaptation that favoured the elaboration of complexity and diversity observed in animal lineages. Coelomates were derived with increased organ-level specialisation and increased mobility that resulted from the selection benefits of their coeloms. They were successful in occupying varied ecological niches and exploiting diverse habitats. In contrast, acoelomates developed reduced body plans suited to less resource-demanding environments. The fossil record and evolutionary studies indeed show how, through graded evolution, coelomates started to advance and diversify from the simpler ancestral forms.
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While coelomates are characterised by a true coelom lined with mesothelium, the acoelomates lack any coelom and their organs are embedded directly into their body wall.
Coelomates include annelids (e.g., earthworms), molluscs (e.g., snails), arthropods (e.g., insects), echinoderms (e.g., starfish), and chordates (e.g., humans). Acoelomates include flatworms (e.g., planarians).
In coelomates, a fluid-filled cavity enables organ specialisation and complex movement, while in acoelomates, organ systems are simple by default and have limited mobility.
Having a coelom provides structural support, facilitates organ development, allows efficient circulation, and provides aid to flexibility and mobility in coelomates.
Coelomates develop their coelomic cavities through processes like schizocoely (protostomes) or enterocoely (deuterostomes), whereas acoelomates lack a true coelom and exhibit simpler embryonic development patterns.
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