Difference Between Acoelomate and Coelomate

Main Difference – Acoelomate vs Coelomate

A group of animals with bilateral symmetry is referred to as bilateria. The bilaterians comprise a head and tail, a back and belly as well as a left side and a right side. Deuterostomia and protostomia are the two divisions of the bilaterians. Protostomia is the group of animals whose blastopore develops into the archenteron. The Protostomia mostly comprises invertebrates with three germ layers. The three divisions of the Protostomia are the acoelomates, pseudocoelomates, and coelomates. Deuterostomia is the group of animals whose blastopore develops into the anus. All Deuterostomia are coelomates. The main difference between acoelomate and coelomate is that acoelomate is an invertebrate that doesn’t have a coelom whereas coelomate is an invertebrate that has a true coelom. A coelom is a fluid-filled body cavity, which is completely lined by the tissues derived from the mesoderm.

Key Areas Covered

1. What is Acoelomate
      – Definition, Characteristics, Examples
2. What is Coelomate
      – Definition, Characteristics, Examples
3. What are the Similarities Between Acoelomate and Coelomate
      – Outline of Common Features
4. What is the Difference Between Acoelomate and Coelomate
      – Comparison of Key Differences

Key Terms: Acoelomate, Coelom, Coelomate, Deuterostomia, Enterocoelom, Haemocoelom, Invertebrates, Mesoderm, Protostomia, , Schizocoelom, Vertebrates

What is Acoelomate

An acoelomate is an invertebrate with three germ layers that lack a body cavity or a coelom. That means acoelomates do not possess a fluid-filled cavity between the body wall and the digestive tract. Therefore, the middle layer of the acoelomates is completely filled with organs and tissues. The middle layer of the body of the acoelomates is derived from the mesoderm. The other two germ layers are the endoderm and the ectoderm. Since acoelomates do not have a coelom, the internal organs, which are derived from the mesoderm, are not protected against the external pressure and shocks. In addition to the coelom, acoelomates also do not comprise a cardiovascular system and a respiratory system. Since acoelomates consist of thin and flat bodies, the gas exchange occurs by simple diffusion. The acoelomates comprise simple organized digestive tracts, nervous, and excretory systems. The elimination of the wastes is achieved through the specialized cells and tubules. A single orifice serves as both the inlet of food and the exit point of wastes. In addition, acoelomates comprise a defined head region with sensory organs to detect light as well as food sources.

Figure 1: A planarian

Platyhelminthes (unsegmented flatworms) are the most precise example of acoelomates. They are free-living animals in the freshwater habitats. Some Platyhelminthes are parasitic. Ribbon worms, tapeworms, flukes, and planarians are examples of Platyhelminthes. The planarian is shown in figure 1.

What is Coelomate

A coelomate is either a triploblastic vertebrate or invertebrate with bilateral symmetry that possesses a true coelom. Coelomates are also called eucoelomates. A coelom is a fluid-filled cavity, which lies between the body cavity and the gut. It develops from the mesoderm. The coelom serves as a cushion to internal organs of the animal body. Moreover, the coelomic fluid found inside the coelom serves as a hydrostatic skeleton. It opens to the exterior through coelomoducts such as oviducts. Based on the formation of the coelom during embryonic development, coelomates can be divided into three types. They are schizocoelom, enterocoelom, and haemocoelom. Schizocoelom is formed by splitting the mesoderm. Mollusks, arthropods, and annelids consist of a schizocoelom. Enterocoelom is formed from the wall of the embryonic gut. Echinodermata and Chordata consist of an enterocoelom. Haemocoelom is a blood-filled cavity, which is found in arthropods and mollusks.

Figure 2: Acoelomates, coelomates, and pseudocoelomates

Coelomates can be found in both protostomes and deuterostomes. Protostomes such as annelids, mollusks, and arthropods are coelomates. Deuterostomes such as Chordata, Echinodermata, Brachiopoda, Ectoprocta, and Phoronida are coelomates. Acoelomates, ceolomates, and pseudocoelomates are shown in figure 2.

Similarities Between Acoelomate and Coelomate

• Most acoelomates and coelomates are invertebrates.
• Both acoelomates and coelomates are triploblastic animals with three germ layers.

Difference Between Acoelomate and Coelomate

Definition

Acoelomate: An acoelomate is an invertebrate that does not possess a coelom.

Coelomate: A coelomate is either a vertebrate or invertebrate that possess a coelom.

Vertebrates/Invertebrates

Acoelomate: Acoelomates are invertebrates.

Coelomate: Coelomates can be either vertebrates or invertebrates.

Protostomes/Deuterostomes

Acoelomate: All coelomates are protostomes.

Coelomate: Coelomates can be either protostomes or deuterostomes.

Mesoderm

Acoelomate: Mesoderm develops into internal organs in the acoelomates.

Coelomate: Mesoderm develops into internal organs and tissues as well as the coelom in coelomates.

Highly Developed Organ Systems

Acoelomate: Acoelomates lack a highly developed organ system.

Coelomate: Coelomates comprise comparatively developed organ systems than acoelomates.

Body Cavity

Acoelomate: The only body cavity of an acoelomate is the digestive cavity.

Coelomate: Coelom and the other cavities in the internal organs along with the digestive tract are the body cavities of coelomates.

Segmentation

Acoelomate: Acoelomates are unsegmented animals.

Coelomate: Coelomates are segmented animals.

Cushioning Effect

Acoelomate: Internal organs are not embedded in a fluid.

Coelomate: Since the internal organs of the coelomates are embedded in the coelomic fluid, the excessive pressure and shocks do not harm to the organs.

Examples

Acoelomate: Ribbon worms, tapeworms, flukes, and planarians like Platyhelminthes are the examples of acoelomates.

Coelomate: Chordata, Echinodermata, Brachiopoda, Ectoprocta, Phoronida, Mollusca, Arthropoda, and Annelida are the examples of coelomates.

Conclusion

Acoelomates and coelomates are two types of triploblastic animals, which differ according to the basic body plan. Most acoelomates and coelomates are invertebrates. Acoelomates do not develop a body cavity or a coelom. In contrast, coelomates develop a fluid-filled coelom from the mesoderm. All deuterostomes are coelomates. Annelids, arthropods, and mollusks are also coelomates. Platyhelminthes is the most precise example of acoelomates. The main difference between acoelomates and coelomates is the presence or absence of a coelom as the body cavity.

Reference:

1. Bailey, Regina. “Acoelomates – Animals Without a Body Cavity.” ThoughtCo. N.p., n.d. Web. Available here. 11 Aug. 2017. 
2. “Coelom: Formation & Types.” Study.com. N.p., n.d. Web. Available here. 11 Aug. 2017. 

Image Courtesy:

1. “Broadhead planarian() 07367” By Vengolis – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Figure 27 02 05” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia