Differences between Coelomate and Acoelomate: Example & Characteristics

What is Coelom?

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 is 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.

Coelomates – Structure, Characteristics & Examples

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 present on all sides, and it surrounds the organs that lie within. This cavity provides cushioning and protection for the organs and gives them room to move independently from the outer body wall.

Characteristics of Coelomates

• Presence of true coelom lined by mesothelium.

• It is within the coelomic cavity in which organs are suspended.

• This allows for development and specialisation of organs.

• It is found in very different groups of animals like annelids, molluscs, arthropods, echinoderms, and chordates.

• Allows for a fast flow of body fluids and provides for a hydrostatic skeleton to be flexible.

Acoelomates – Structure, Characteristics & Examples

An acoelomate 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.

Characteristics of Acoelomates

• 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.

Comparison Table – Coelomates vs Acoelomates

Coelomates are animals with a fluid-filled body cavity lined with mesoderm, and allows organs to be protected and move freely. Acoelomates lack 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.

Structure and Formation of Coelom

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 and by enterocoely in deuterostomes. Fluid-filled spaces are formed and organs are present inside these spaces.

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 Differences Between Coelomates and Acoelomates

Physiological adaptations linked with coelomates and acoelomates are significantly different. In most cases, the digestive system of coelomates is 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 the waste products are expelled.

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.

Functional and Evolutionary Significance of Coelom

The coelomates 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 evolution of highly specialised organs or complex respiratory tract and digestive systems and sophisticated sensory organs. Acoelomates lack coelom, and instead 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 in coelomates, is a strong adaptation that favoured the complexity and diversity observed in animal lineages. Coelomates were derived with increased organ-level specialisation and increased mobility that resulted from the natural 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.

Coelomates and Acoelomates NEET MCQs

Q1. The pericardial cavity, pleural cavity, and peritoneal cavity are

1. Germ Layers

2. Coelom

3. False Coelom

4. None of the above

Correct answer: 2) Coelom

Explanation:

The pericardial, pleural, and peritoneal cavities are vital spaces in the ventral cavity of humans and other vertebrates that encase significant organs.

Pericardial Cavity:
- Located in the thoracic cavity, it envelops the heart.
- Composed of a pericardium with inner visceral and outer parietal layers.
- Holds pericardial fluid to minimize friction during cardiac contractions.

Pleural Cavity:
- Consists of two compartments within the thoracic cavity, each enclosing a lung.
- Pleura forms a dual-layered structure (visceral on the lung, parietal on the cavity wall).
- Contains pleural fluid for lung lubrication during respiration.

Peritoneal Cavity:
- Situated in the abdominopelvic cavity, it encompasses the abdominal viscera.
- Lined with the peritoneum, a serous membrane.
- Holds peritoneal fluid to lessen friction during organ movement and digestion.

Hence, the correct answer is option 2) Coelom.

Q2. Select the one which is not correctly matched:

1. Coelomates - Annelids

2. Pseudocoelomate - Aschelminthes

3. Coelomates - Coelenterates

4. Acoelomate - Platyhelminthes

Correct answer: 3) Coelomates - Coelenterates

Explanation:

The match "Coelomates - Coelenterates" is incorrect because coelenterates, which belong to the phylum Cnidaria, are diploblastic organisms with only two germ layers and lack a true coelom. Instead of a coelom, they possess a gastrovascular cavity that functions as both a digestive and circulatory chamber. Coelomates, on the other hand, are animals with a true coelom, a body cavity completely lined by mesodermal tissue, seen in organisms such as annelids, mollusks, arthropods, echinoderms, and chordates.

Hence, the correct answer is option 3) Coelomates - Coelenterates

Q3. Bilaterally symmetrical and acoelomate animals are exemplified by

1. Ctenophora

2. Platyhelminthes

3. Aschelminthes

4. Annelida

Correct answer: 2) Platyhelminthes

Explanation:

Bilaterally symmetrical and acoelomate animals are exemplified by Platyhelminthes. Platyhelminthes (flatworms) are bilaterally symmetrical and acoelomate animals, meaning they lack a body cavity between the gut and the outer body wall. Their flat, ribbon-like body structure allows for efficient diffusion of gases and nutrients directly across the body surface. These animals often have simple organ systems, including a nerve ladder, but lack specialized circulatory and respiratory systems due to their small size and acoelomate condition.

Hence, the correct answer is option 2 (Platyhelminthes).

Also Read:

• MCQs on Coelom

• Difference Between Chordates and Non-chordates

• Osteichthyes

FAQs on Coelomates and Acoelomates

What is a coelom?

A coelom is a fluid-filled body cavity that lies between the body wall and the digestive tract. It is completely lined by a layer of mesodermal cells called mesothelium. It serves as a space where internal organs such as the heart, kidneys, and lungs can develop and remain suspended. The presence or absence of a coelom is an important basis for animal classification.

What are the differences between coelomates and acoelomates?

• Coelomates (Eucoelomates): Animals having a true coelom fully lined with mesoderm. This cavity allows well-developed organ systems and efficient circulation. e.g., annelids, molluscs, arthropods, echinoderms, chordates

• Acoelomates: Animals that lack coelom and instead space between the gut and body wall is filled with mesenchyme or parenchyma. e.g., Platyhelminthes or flatworms.

What is the function of coelom?

The coelom has following functions:

• acts as a cushion protecting internal organs from mechanical shocks

• allows movement of internal organs from the body wall

• serves as a hydrostatic skeleton in soft-bodied animals (e.g., annelids)

• provides space for circulation of coelomic fluid which distributes nutrients

What are methods for coelom formation?

• Schizocoelous formation: The mesoderm first forms as solid masses between the ectoderm and endoderm, which then split to form the coelomic cavity. Examples include protostomes such as annelids, molluscs, and arthropods.

• Enterocoelous formation: The mesoderm originates as pouches from the wall of the embryonic gut (archenteron), and these pouches pinch off to form the coelom. Examples include deuterostomes such as echinoderms and chordates.

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