Origin and Organization of Coelom_MS.pptx

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.

3. 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

4. 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

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

6. 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 .

7. 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

8. 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

9. THEORIES OF ORIGIN AND EVOLUTION OF COELOM Schizocoel theory - Clark, 1964 According to this theory, the coelomates evolved from an ancestral acoelomate like flatworms by hollowing out of the parenchymal cells of the mesenchyme . Some of these cells formed the peritoneum. According to this theory, the acoelomate body plan is primary and ancestral to the coelomate plan . The acoelomate flatworms, thus, form the basic group in the evolution of bilateral animals. The schizocoel mode of coelom formation in the embryonic development of annelids and mollusks would claim as supporting evidence of this theory. However, the evolution of coelom is not related to gonads or endodermal pouches of lower forms.

10. Enterocoel theory   First proposed by Lankester in 1877, supported by Lang (1881), Sedgwick (1884): This theory states that the coelom may have originated by evagination as pouch-­like structures in the wall of embryonic arch­enteron . This type of coelom formation oc­curs in many existing enterocoelous animals. This theory proposes that all bilateral animals are basically coelomate and that acoelomate forms like flatworms are secondarily derived from coelomate ancestors by the loss of the cavity. The enterocoelous mode of coelom formation in the embryogeny of echinoderms, hemichordates and chordates is the main supporting evidence of this theory .

11. Gonocoel theory HatSchek , 1877, 1878), Bergh (1885), Meyer (1890), Goodrich (1946 ) The origin of coelom in favour of gonocoel theory is that first coelomic cavities arose from the mesodermally derived expanded gonadal cavities and the cavities persisted after the release of gametes . For example, the gonads of tricladid flatworms are arranged in a linear order and the segmental coelom of annelida may have developed from this tricladid . One of the main drawbacks of this theory is that it closely links the origin of coelom with the origin of metameric segmentation and hence it is difficult to account for the unsegmented coelomates. There is no evidence that the unsegmented coelomates have originated from the segmented ancestors. This theory has no embryological support because gonads do not arise before the coelom.

12. Nephrocoel theory   Proposed by Lankester in 1874 & Snodgrass , 1938. The coelom originated as an expanded nephridia of flatworms. This theory however, was never taken seriously because protonephridia has been described in coelomates and also excretory organs are absent in some coelomates like echinoderms.