Differences between Coelomate and Acoelomate: Example & Characteristics

What are Animal Body Plans?

Diversity in animal body plans thus reflects evolutionary adaptations that enable different species to succeed in these very different environments. One of the key features of animal body plans is the presence or absence of body cavities, which impacts enormously on the structure and function of an animal.

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This Story also Contains

1. What are Animal Body Plans?
2. Coelomate
3. Acoelomate
4. Table: Difference Between Coelomates and Acoelomates
5. Structure and Formation of Coelom
6. Physiological Differences Between Coelomates and Acoelomates
7. Functional Significance
8. Evolution of Body Cavities
9. Ecological and Environmental Roles

Body cavities are spaces within the body of an organism that shelter organs and can offer a framework for organ systems. Knowledge of these cavities is instrumental in understanding the diversity and functionality of various animal groups. Two main groups into which the diversified body plans can be divided based on body cavities are coelomates and acoelomatous animals.

Coelomate

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 encapsulates 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. Essentially, this means an increase in organ-system complexity of coelomates due to the allowing of spatial separation by the coelom.

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Characteristics of Coelomates

• Presence of true coelom lined by mesothelium.

• It is within the coelomic cavity that suspension of the organs takes place.

• This allows for a more developed and intricate specialization 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 mobile.

Acoelomate

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.

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.

• Little room for organ specialisation and movement.

• Only relies on the process of diffusion for the distribution of nutrients and waste products.

Table: Difference Between Coelomates and Acoelomates

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 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.

Diagram: Basic Structures of Coelomate and Acoelomate Animals

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 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, just like their 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.

Functional Significance

The coelomates do have a fluid-filled coelom that provides them with a hydrostatic skeleton, enhancing 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.

Evolution of Body Cavities

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 active selective 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.

Ecological and Environmental Roles

Coelomates and acoelomates occupy different ecological niches based on their structural adaptations. Coelomates are found occupying almost every habitat, from the depths of the ocean to land, equipped with advanced body plans and specialised organ systems. In contrast, acoelomates have less complex body structures and usually occupy more stable environments where simple physiological functioning is sufficient. Those ecological roles will also help understand the contributions toward ecosystem stability and biodiversity, making the evolution of the body cavity of foremost importance in shaping ecological interactions and adaptations to the environment.

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