Describe the coelom and its types

1. ORIGIN AND ORGANIZATION OF COELOM
Dr. MADHUSUDHAN

2. COELOM
• The term coelom was suggested by Haekel in 1872.
• Coelom is defined as a fluid filled perivisceral cavity lined by epithelial cells
derived from the embryonic mesoderm.
• The body cavity in most animals, located between the gut (intestinal canal) and
the body wall.
• Coelom arises as a secondary body cavity between two embryonic layers of
mesoderm.
• Coelom is completely lined by tissue created from the mesoderm, the middle
layer of the primary cells found in an embryo. It contains most of the visceral
organs.
Features of a True Coelom
• It is a secondary body cavity formed by the splitting or evagination of mesoderm
during embryonic development.
• It is also bounded on all sides by a definite coelomic epithelium.
• It consists of colourless coelomic fluid also the excretory organs open into it.
• Reproductive organs arise from its walls.

4. • The coelom is lined externally by a parietal epithelium and internally by a visceral or splanchnic
epithelium.
• The peritoneum surrounds all the internal organs, including the alimentary canal. In other words, all
the internal organs are present behind the peritoneum. Thus, coelom is also called as the perivisceral
cavity.
• Coelom is divided into fluid filled compartments by transverse partition called septa which extend from
the annuli of the body wall to the alimentary canal.
• The wall of the septa is perforated, through which coelomic fluid communicates from one
compartment to another. The coelom communicates with the exterior by two sets of ducts namely
nephridia and coelomoduct.
• Nephridia are ectodermal tubes that remove water and excretory waste while coelomoduct are
mesodermal tubes which usually open into the coelom at one end while the other end communicates
with the exterior.
• However, in some coelomates like arthropods and mollusks, the coelom is reduced or absent in adult,
but is present in the embryo. The space containing blood and lymph is in the form of tubes through
which fluid is made to circulate by the heart, and this space is called a haemocoel. The visceral
organs are surrounded by the haemocoel. The haemocoel reduces the coelom to small cavities
around the heart, gonads and kidney. In these animals, the haemocoel is called the primary body-
cavity while the coelom is called the secondary body cavity.
COELOM

5.  In organisms such as worms, the coelom acts to maintain pressure inside of the
organism. This acts as the worm's skeletal system, which allows the worm's muscles to
do work.
 Coelom surrounds the internal organs and thus protects them from external shocks.
 It serves as a hydrostatic skeleton to assist in locomotion and maintaining shape of the
body.
 It provides flexibility to the body. The internal organs in coelomate animals become large
and are able to perform movements freely of their own.
 It helps in removing excretory waste from the body.
 Helps in the transportation of gases and nutritive materials from one part of the body to
the other.
 In some animals, it keeps the body wall moist to facilitate respiration and to destroy
bacteria and other harmful micro-organisms.
 Acts as a site for gamete maturation and brooding of embryos.
 In mammals coelom forms into the peritoneal, pleural, and pericardial cavities.
 The presence or absence of coelom is important for the classification of animal phyla.
FUNCTIONS OF COELOM

6. Acoelomata: Acoelomates do not have a coelom or body cavity. The
space between the body wall and gut is filled up by mesenchyme and
Eg: phylum Porifera, Coelenterata, Platyhelminthes and Nemertinea
Pseudocoelomata: The space between the body wall and gut encloses
a fluid filled cavity which is not lined by mesodermal epithelial cells.
pseudocoel or false coelom and the animals possessing it are known
Embryologically, the pseudocoelom is derived from the blastocoel of
organs are free within the space, as there is no peritoneum bounding
Eg: Phylum Nematoda, Acanthocephala, Entoprocta, Rotifera, and
Coelomata: In rest of the bilateria, the body-cavity present between
the body wall and gut is lined by a peritoneum. Peritoneum consists
by embryonic mesoderm. This type of cavity is known as true coelom
possessing it are known as Eucoelomates
Eg: phylum Annelida, Arthropoda, Mollusca, Echinodermata,
Organization of Bilateria based on coelom
TYPES OF COELOM

9. COEMOMATES [ EUCOEOLMATES]
• The cavity that is present in between the body wall and the
alimentary canal is lined on either side by epithelial layers is
called Eucoelom or true coelom. It is lined by means of
mesoderm.
• The coelomic epithelium present below the body wall is reffered
as partial layer or somatic layer.
• The coelomic ephithelium present above the gut wall is reffered
as visceral layer or Splachnic layer. Both the layers are
mesodermal in origin.
• Based an the mode of formation of coelom, coelomates are
classified into two types; they are
1) Schizocoelomates
2) Enterocoelomates.

10. • Schizocoelomates: The cavity formed by the splitting of embryonic mesoderm is called
‘Schizocoelom’, and these animals are referred as Schizocoelomates.
E.g. :Annelida, Arthropoda , Mollusca.
• In Schizocoelomates the Zygote exhibits spiral cleavage.
• Embryonic mesoderm is formed through Teloblastic method.
• In this method a single micromere called 4d blastomere or mesentoblast cell present at
the rim of blastomere or mesentoblast cell present at the rim of blastopore proliferates to
form mesoderm between the developing archenteron( endoderm) and the body wall (
ectoderm), in the blastocoel.
• Generally a series of poured blocks of mesoderm develops. A split appears within each
block making them hallow.
• The two bands come into contact below the body wall and above the alimentary canal.
Thus Parietal layer below the body wall and visceral layer above the alimentary canal are
formed with a true coelomic space at the middle of the layers.
In Arthropods and molluscs Schizocoelom is reduced and the functional body cavity is
haemocoel.
SCHIZOCOELOM / SCHIZOCOELOMATES

12. • In Enterocoelomates the lumen from the alimentary canal
extends in to the mesoderm in the form of pouches to form
coelom. So the coelom is referred as enterocoelom.
Ex: Echinoderms, hemichordates and chordates.
• In the embryonic condition, archenteron produces a pair of
lateral pouches.
• These pouches get pinched off from the archenteron into the
blastocoel.
• The cavity within these pouches is the enterocoelom. As these
pouches enlarge and fuse, the blastocoel is replaced into
somatic layer below the body wall and splachnic layer above
the alimentary canal.
ENTEROCOELOM / ENTEROCOELOMATES

15. THEORIES OF ORIGIN AND EVOLUTION OF
COELOM
Schizocoel theory - Clark, 1964
• According to this theory, the coelomates evolved from
acoelomate like flatworms by hollowing out of the
mesenchyme. Some of these cells formed the
this theory, the acoelomate body plan is primary and
coelomate plan.
• The acoelomate flatworms, thus, form the basic group
bilateral animals. The schizocoel mode of coelom
embryonic development of annelids and mollusks
supporting evidence of this theory. However, the
related to gonads or endodermal pouches of lower

17. Enterocoel theory
• First proposed by Lankester in 1877, supported
Sedgwick (1884):
• This theory states that the coelom may have
evagination as pouch-like structures in the wall
archenteron. This type of coelom formation oc-
existing enterocoelous animals.
• This theory proposes that all bilateral animals
coelomate and that acoelomate forms like
secondarily derived from coelomate ancestors
cavity. The enterocoelous mode of coelom
embryogeny of echinoderms, hemichordates
the main supporting evidence of this theory.

19. Gonocoel theory
HatSchek, 1877, 1878), Bergh (1885), Meyer (1890),
• The origin of coelom in favour of gonocoel theory is that
cavities arose from the mesodermally derived expanded
and the cavities persisted after the release of gametes.
• For example, the gonads of tricladid flatworms are
order and the segmental coelom of annelida may have
tricladid.
• One of the main drawbacks of this theory is that it
coelom with the origin of metameric segmentation and
account for the unsegmented coelomates. There is no
unsegmented coelomates have originated from the
This theory has no embryological support because
before the coelom.

21. Nephrocoel theory
• Proposed by Lankester in 1874 & Snodgrass,
• The coelom originated as an expanded
flatworms.
• This theory however, was never taken
protonephridia has been described in
excretory organs are absent in some
echinoderms.