Coelomate Invertebrates, Biology tutorial


Coelomates are animals which contain coelom that is fluid-filled body cavity between 2 layers of mesoderm in some triploblastic metazoans. 3 body plans of metazoans are distinguishable on basis of the:

a) Absence of the body cavity (acoelomate)

b) Occurrence of the false body cavity (pseudocoelomate) and

c) Occurrence of the true body cavity (coelomate/eucoelomate).

Invertebrates are usually referred to as animals without a backbone. This definition generally comprises single-celled protozoans (animal-like protoctistans) and all other phyla of animals which don't have vertebral column. When animals are studied and compared, several are found to contain row of internal bones along centre of the back; these bones are known as vertebrae and animals which have them, vertebrates. Vertebrates comprise reptiles, amphibians, birds, mammals and fishes, and they make up approx 3% of animal kingdom.

The other 97% are animals without backbones or invertebrates. There are several invertebrate animals which catch the attention while several of them go unnoticed because of their microscopic size or as they live outside general view of humans (like in water, underground, in soil, etc.).

Body cavities of animals:

Though body cavity can be any internal body space, generally, it refers to fluid-filled space between internal organs and body wall. Invertebrate phyla Cnidaria/Coelenterata and Platyhelminthes possess a solid kind of body, with mesogloea and mesenchyme/parenchyma respectively, between ectoderm and endoderm. All other phyla higher than the two contain body cavity which divides gut from body wall.

Types of body cavities:

Two types of body cavities take place in animals, less common is pseudocoelom and most common is coelom. Both cavities are created during embryonic development.


First cavity to emerge during embryonic development is blastocoel, at times known as primary body cavity. Formation of the gut decreases blastocoel, but some of space between endoderm and ectoderm remains. It is in this part of body (blastocoel) which solid parenchyma of acoelomates forms. Pseudocoel is the cavity left by failure of parenchyma to entirely fill space between mesoderm of body wall and gut wall; it is the derivative of blastocoel. Pseudocoel takes place between mesoderm, which creates muscle of body wall, and gut wall that is composed of endoderm. Gut wall in pseudocoelomates thus has no muscle layers as in coelomates. Pseudocoelom is found in one major phylum, Nematoda, and few minor phyla, Nematomorpha, Gastrotricha, Acanthocephala Rotifera and Kinorhyncha.


Coelom is the secondary fluid-filled body cavity which creates as new space in mesoderm and is entirely enclosed by mesodermal tissue. Fluid in coelomic cavity isn't in direct contact with either gut or body wall, but is divided from both by peritoneal epithelium (peritoneum). Coelom forms in development in some animals by dividing of mesoderm because of accumulation of fluid. As coelom increases in size, outer part of mesoderm becomes intimately related with body wall, and body wall is lined by parietal or somatic mesoderm which becomes parietal peritoneum. The kind of coelom formed by dividing of mesoderm is known as a schizocoel. Haemocoel develops from cavities of blood vascular system and is thus filled with blood.  Coelom still exists but it is confined to cavities of excretory organs and reproductive ducts. High blood volume to body volume in arthropods allows maintenance of high metabolic rate, permitting them to have high level of activity. Coelom can also be formed from out-pocketings/ outpouchings of the mesoderm. Coelom created in this manner is called as enterocoel. It happens in echinoderms, hemichordates and chordates.


This is the significant structural adaptation observed in several coelomates. It is division of body in series of similar segments which are developed to same degree and arranged in the linear series from anterior to posterior end of animal. Every segment may contain sub-divisions of organ systems. The ideal metamerically segmented animal would contain each segment with reproductive and excretory organs, and skin, nerves, muscles, and circulatory structures.

The importance of Coelom:

i) Organs which are embedded in solid tissue (parenchyma) as in acoelomates are compressed any time animal moves its body while there is freedom of movement of internal organs in animals which have a body cavity. This freedom of movement permits for effective transportation of gut contents.

ii) The body cavity gives space for growth of gonads so that eggs and sperm are stored in the constant environment. Benefit of this is that animals with the body cavity can limit the breeding to certain times of year which are favorable, unlike those which breed all year round.

iii) Fluid in body cavity acts as the transporting medium for metabolites and excretory products. Fluid-filled cavity can also act as the hydrostatic skeleton. The hydrostatic skeleton is in contact with muscles of body wall and gut wall with increased mechanical effectiveness and rapidity of movement.

iv) Increase in size and complexity is probable in animals with the body cavity. Problems which might happen with increase in size are solved by the transport system like a blood vascular system.


This refers to development of all characteristics related with head. Development of feeding apparatus, sense organs and nervous tissue are all element of cephalization.

Protostomes and Deuterostomes:

The coelomates are separated in 2 large groups. Though there are exceptions to the given generalizations, most zoologists are convinced that two groups are true evolutionary groups.

i) Protostome embryonic development which characterizes annelids, molluscs, and arthropods; comprises 4 main components:

  • Spiral cleavage of egg after fertilization. Upper tier of cells is twisted 45° out of line with lower tier.
  • Blastopore grows in mouth (proto = first, stome = mouth). Indentation on surface of developing embryo in some animals by which cells from surface pass in embryo to take their positions for further growth.
  • Schizocoelous - coelom formed by splitting of the mesoderm.
  • Determinate (Mosaic) Embryo - removing cell from 4-cell stage results in arrested development of embryo. Eggs whose development seems to be directed by cytoplasmic determinants localized in different parts of egg are said to be mosaic eggs. Cells formed at early pahases of development grow independently of each other and are not able to substitute for each other if one is removed.

ii) Deuterostome - embryonic development which characterizes echinoderms and chordates; comprises 4 major components which differ from protostomes.

  • Radial Cleavage - cleavage of fertilized egg is radial. The upper tier of cells sits directly on top of the lower tier
  • Blastopore develops into the anus (deutero = second, stome = mouth)
  • Enterocoelous - coelom formed from outpouching/outpocketting of the mesoderm
  • Indeterminate (Regulative) Embryo - removing the cell from the embryo doesn't influence development; fate of the cells of zygote is not prearranged.

Description of some terms associated to development of Coelom:

Levels of Organization:

Cytoplasmic/protoplasmic - all life tasks are performed within limits of single cell membrane

Cellular - several different functions are performed by specific cellular structures

Tissue - similar cells collected together to carry out common functions as highly coordinated unit

Organ - tissues are collected in these larger functional units

Organ System - different organs operate together to carry out vital functions

Germ Layers:

Diploblastic: simplest level of tissue organization, comprising of 2 embryological layers: (i) ectoderm that gives rise to epidermis (outer layer of body wall) and (ii) endoderm that gives rise to gastrodermis (tissue which lines gut cavity)

Triploblastic: more complex level of tissue organization, comprising of three embryological layers: (i) ectoderm; (ii) endoderm and (iii) mesoderm (iii is between ectoderm and endoderm; gives rise to the organs of circulation, excretion, and reproduction plus the muscles, connective tissue, and blood cells.)

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