Difference Between Coelomate and Acoelomate

Coelomate vs Acoelomate

Classification of animals is mainly done based on the morphological characteristics expressed by them. Presence and absence of a body cavity, as well as, the type of it have provided the scientists with major criterion for classification. A body cavity can be plainly defined as a cavity, which lies between the epidermis/ body covering and the outer covering of the gastrointestinal cavity. Depending on the type of the body cavity, animals can be grouped as coelomic, pseudocoelomic and acoelomic. This difference in the type of the body cavity, even it doesn’t correlate with any evolutionary relationship, has given rise to many other differences between coelomates and acoelomates. 

Who are Coelomates? 

Coelomates possess a fluid filled body cavity/true coelome, which is completely lined by the mesoderm-derived epithelium. All the body organs are suspended within this and have a greater degree of freedom to develop, grow and move independently from the body wall while maintaining more organized structure within the body. Additionally, this type of body cavity provides protection to those delicate, suspended organs by acting as a shock absorber. Fluid filled coelome cavity also acts as a hydrostatic skeleton, which gives support and some sort of a shape, especially to those animals without hard skeletal structures. Apart from those, coelomic fluid act as a transport medium to many soluble gases and metabolites, which is a greater advantage for animals with complex body forms with low surface to volume ratio. Vertebrates and most of the other animals that have bilateral symmetry can be categorized as coelomates. 

Who are Acoelomates? 

Acoelomates as the name implies, lack a fluid filled body cavity completely lined with the mesoderm. Their body organs are embedded within the mesoderm derived paranchymal tissues rather than suspended, limiting their freedom to move, develop and grow independently. Lack of a fluid filled cavity also removes the advantage of incompressible nature of fluid; thus, making the body organs and the whole body vulnerable to external mechanical pressure. Lack of a fluid filled body cavity can be considered as one of the major reasons for the rather simple body plan of acoelomic animals. Many acoelomates are dependent on simple diffusion for essential gases and metabolites supply and must have flatter body shapes to increase surface to volume ratio. Flat worms/phylum platyhelminthes can be given as an example for acoelomates. 


What is the difference between Coelomates and Acoelomates? 

• Acoelomates lack a fluid filled body cavity, which is completely line by the mesoderm, as opposed to coelomates.

• Organs of a coelomate are suspended in the coelomic fluid, and in an acoelomate, body organs are embedded within the mesoderm-derived parenchyma.

• Suspended organs of a coelomate can develop, grow and move independently, than that of an acoelomate.

• Incompressible fluid nature of the coelomic fluid protects the animal against any external mechanical pressure, unlike in acoelomates

• Coelomates that lack a hard skeleton gain support from the hydrostatic skeleton produced by the coelomic fluid, but acoelomates do not have such advantage.

• Acoelomates that lacks a fluid filled cavity, has lost the advantage of it as a transport media, and they have to maintain body shapes essentially for efficient gaseous exchange from outside, unlike the coelomates.

• Since coelomates do not depend on simple diffusion for metabolites and gaseous exchange, they have evolved complex body forms compared with the acoelomates.