Difference Between Coelom and Haemocoel

The key difference between coelom and haemocoel is that the coelom is the principal body cavity of annelids, echinoderms and chordates that originated from the mesothelium while haemocoel is the primary body cavity of arthropods and molluscs which is a reduced form of a coelom.

Most multicellular animals possess body cavities filled with fluids that surround their organs. These cavities have various functions. The body cavity inside the body is commonly called coelom. However, not all organisms possess a coelom. Coelomates are the organisms that have a coelom. The organisms that do not possess a coelom are acoelomates. Porifera and Platyhelminthes are acoelomates. Generally, mesoderm lines the true coelom; hence it is mesodermal. However, certain organisms possess a coelom called pseudocoelom, which is not mesodermal. Haemocoel is another type of primary body cavity found in some organisms called haemocoelomates. Therefore, coelom and haemocoel are two types of body cavities present in different classes of animals. This article analyses the difference between coelom and haemocoel briefly.

CONTENTS

1. Overview and Key Difference
2. What is Coelom
3. What is Haemocoel
4. Similarities Between Coelom and Haemocoel
5. Side by Side Comparison – Coelom vs Haemocoel in Tabular Form
6. Summary

What is Coelom?

Coelom is a true perivisceral cavity that develops within the mesoderm during the embryonic development of triploblastic animals. Generally, coelom separates mesoderm into two parts:  one part is in contact with the ectoderm while the other part is in contact with endoderm. In addition, the coelomic cavity contains a fluid called coelomic fluid.  A layer of mesodermal cells called peritoneum secretes this coelomic fluid.

Figure 01: Coelom

Certain groups of animals possess a true coelom, and they are the coelomates.  Therefore, annelids, echinoderms, and chordates are coelomates, and they have a true coelom, which is of mesothelium origin.

What is Haemocoel?

Haemocoel is a type of primary body cavity present in arthropods and molluscs. It contains blood or haemolymph composed of haemocytes and colourless plasma. Two types of haemocytes are proleucoeytes and phagocytes. Haemolymph in haemocoel mainly serves as a source of distributing and collecting nutrients and metabolic wastes, and it functions as a lymph tissue. Haemocoel surrounds all internal organs. Two horizontal septa called diaphragm and ventral diaphragm separate haemocoel into three blood sinuses: pericardial sinus, perivisceral sinus, and perineural sinus.

What are the Similarities Between Coelom and Haemocoel?

• Both coelom and haemocoel are body cavities.
• They are filled with fluids.
• They act as a cushion and protect internal organs.
• Furthermore, both coelom and haemocoel allow internal organs to grow, develop, and change over time.
• Moreover, both act as a hydrostatic skeleton.
• Besides, both are present only in the organisms of Kingdom Animalia.

What is the Difference Between Coelom and Haemocoel?

Coelom is the main body cavity that surrounds the gastrointestinal tract and other organs of annelids to chordates while haemocoel is a primary body cavity that contains the circulatory fluid of arthropods and molluscs. So, this is the key difference between coelom and haemocoel. Additionally, a further difference between coelom and haemocoel is that the coelom contains coelomic fluid while haemocoel contains haemocoelomic fluid. Moreover, organisms that have a coelom are called coelomates while organisms that have a haemocoel are called haemocoelomates.

Besides above differences, one other difference between coelom and haemocoel is that the coelom is a secondary body cavity while haemocoel is a primary body cavity. In addition, coelom is lined by coelomic epithelium while haemocoel is lined by the basal lamina of the epithelial sheet. Therefore, this is also a difference between coelom and haemocoel.

The following infographic explains the difference between coelom and haemocoel in more detail.

Summary – Coelom vs Haemocoel

Coelom is a fluid-filled main body cavity present between the intestinal canal and the body wall of echinoderms and chordates. It is lined with mesodermal epithelium. On the other hand, acoelomates lack a true coelom. Haemocoel is another type of a primary body cavity filled with circulatory fluid. Blood circulates through the haemocoel. It is a reduced form of a coelom. Thus, this is a summary of the difference between coelom and haemocoel.

Reference:

1. “Coelom: Formation & Types,” Study.com, Available here.
2. “Coelom – Definition and Examples of Coelomates.” Biology Dictionary, Biology Dictionary, 8 June 2017, Available here.

Image Courtesy:

1. “Annelid redone w white background” By KDS444 – Own work (CC BY-SA 3.0) via Commons Wikimedia

ncG1vNJzZmivp6x7pbXFn5yrnZ6YsqOx07CcnqZemLyue8OinZ%2Bdopq7pLGMm5ytr5Wau26vzp6jqKVdlruledWsZKGZlaK8pLvEpWY%3D