Annelida: Coelom | Zoology Optional Notes for UPSC PDF Download

Coelom Evolution and Types in Animals

• Embryonic Origin: Coelom originates as a split in the mesoderm, dividing into two layers: the somatic layer near the epidermis and the splanchnic layer around the endoderm.
• Coelomic Epithelium: Coelom is bounded by coelomic epithelium, which secretes coelomic fluid.
• Perivisceral Cavity: The majority of the coelom forms the perivisceral cavity or splanchnocoel, housing the viscera. This arrangement allows the viscera to remain independent of body wall muscle movements.
• Compartmentalization: In some higher animals, a portion of the perivisceral cavity forms restricted coelomic cavities. This coelomic nature becomes apparent only through developmental history.
• Ancestral Segmentation: Ancestral coelomate animals likely had segmentally arranged mesodermal pouches.
• Gamete Formation: From these pouches, gametes were formed through epithelial lining proliferation.
• Modern Relevance: The evolution of mesodermal pouches is evident in present-day coelomates.

Types of Coelom

• Acoelom: No coelom, and the space between the gut and body wall is filled with densely packed connective tissue derived from both ectoderm and endomesoderm.
• Pseudocoelom: A fluid-filled body cavity between the gut and the outer body wall musculature, generally formed by the persistence of the embryonic blastocoel. Not derived from embryonic mesoderm and lacks a coelomic epithelium.
• Coelom or Eucoelom: A true coelom lying between the gut and the outer body wall musculature, lined by coelomic epithelium derived from embryonic mesoderm. Opens to the exterior through coelomoducts, containing amoeboid cells or amoebocytes.

Mode of Coelom Formation

• Schizocoely: Coelom arises through the splitting of mesodermal bands or masses during embryonic development. Examples include Mollusca, Sipuncula, Echiura, Priapulida, Annelida, Arthropoda, Tardigrada, and Onychophora.
• Enterocoely: Coelom is formed by the evagination from the embryonic archenteron. Pouch-like structures detach from the archenteron and gradually occupy the entire body. Examples include Echinochordata, Hemichordata, and Chordata.

Coelomates: Protostomes and Deuterostomes

Embryonic Development

Coelom Development

• In protostomes, the coelom forms through the hollowing out of mesoderm in blocks.
• In deuterostomes, the coelom develops from mesoderm pockets near the endoderm, which pinch off to create the coelom.
• Despite different developmental paths, both protostomes and deuterostomes end up with a simple, spherical embryo with a digestive system, featuring a mouth at one end and an anus at the other.

Coelom in Different Animal Groups

• Sipuncula: These organisms have two coelomic cavities, a tentacular coelom located at the base of the tentacles and extending into three branches in each tentacle, and a trunk coelom occupying the trunk region.
• Echiura: Similar coelomic structures are found, with a spacious and uninterrupted trunk coelom. Coelomic fluid circulates via muscular contractions and cilia of the coelomic lining.
• Priapulida: The nature of the body cavity, whether it's a pseudocoelom or a coelom, is unclear. The body cavity fluid contains amoebocytes and erythrocytes.
• Pogonophora: The coelom is compartmentalized and extends into the tentacles, with the coelomic fluid containing respiratory pigment and hemoglobin.
• Onychophora: The primary body cavity is a haemocoel rather than a true coelom, with the true coelom restricted to gonadal cavities and excretory organs.
• Mollusca and Arthropoda: In some species, the cavities of the blood vascular system become enlarged, obliterating the perivisceral coelom. This results in a spacious cavity filled with blood, known as a haemocoel.
• Annelids: These organisms have coelomic vesicles between each segment of the gut and the corresponding body wall segment, lined by peritoneum. The annelid body also features dorsal mesentery, ventral mesentery, and transverse septa.
• Echinodermata: The adult echinoderms have several distinct coelomic spaces. These spaces develop as a pair of lateral pouches, which become the future coelomic cavity. The cells forming the pouch wall transform into mesoderm.

Coelom Formation Theories

• Enterocoel Theory: Suggests that coelom originates from evagination as pouch-like structures in the wall of the embryonic archenteron.
• Gonocoel Theory: Proposes that the first coelomic cavities arise from expanded gonadal cavities.
• Nephrocoel Theory: Suggests that the coelom originates from the expanded nephridia of flatworms.
• Schizocoel Theory: Argues that the coelom evolves through the splitting of mesodermal plates.

Significance of Coelom

The coelom plays a crucial role in animal life:

• Facilitates smooth transportation of materials in solution.
• Provides flexibility to the body and allows the movement of the gut, which remains suspended.
• Houses gonads and nephridial tubules.
• Acts as a hydrostatic skeleton and aids in locomotion through incompressible coelomic fluid.