The body cavity, which is lined by mesoderm, is called coelom more elaborately, coelom is a fluid-filled space between the body wall and visceral organs and lined by mesodermal epithelium peritoneum.
Based on the body cavity-triploslastic animals can be classified into acoelorrentes pseudo-coelo mates, and coelomates.
Acoelomate bilaterians : The bilaterian animals in which the body cavity is absent are called acoelomates, e.g. Platy-helminthes lowest bilaterians). In these animals, the mesenchyme derived from the thrid germinal layer, called mesoderm, occupies the entire blasto coel, between the ectoderm and the endoderm, so that the adults have neither the primary cavity (blasto-coelom) nor the secondary cavity (coelom). As there is no body cavity, the acoelomates exhibit solid body plan.
Pseudocoelomate bilaterians : In some animals, the body cavity is not lined by mesodermal epithelia. Such animals are called Pseudocoelomates. They include the members of phylum Aschelminthes (Nematoda, Rotifera and some minor phyla). During the embryonic development mesoderm (mesenchyme) occupies only a part of the blastocoel adjoining the ectoderm. The unoccupied portion of the blastocoel persists as pseudocoelom.
Eucoelomate bilaterians : Coelom or true coelom. is a fluid-filled cavity, that lies between the body well and the visceral organs and is lined by mesodermal epithelium, the peritoneum. The portion of the peritoneum that underlines the body wall is the parietal peritoneum or somatic peritoneum. The portion of the peritoneum that covers the visceral organs is the splanchnic peritoneum or visceral peritoneum. In coelomates, the visceral organs are suspended in the coelom by the peritoneum.
During the embryonic development of the eucoelomates, the blastocoel is replaced by true coelom derived from the mesoderm. So, the true coelom is also called .secondary body cavity. Based on the mode of formation of coelom, the eucoelomates are classified into two types :
I. Schizocoelomates : Animals in which the body cavity is formed by .splitting of mesoderm. are called schizocoelomates. Annelids, arthropods and molluscs are schizocoelomates in the animal kingdom. All the schizocoelomates are protostomians and they show .holoblastic., .spiral and .determinate. cleavage. The 4d blastomere or mesentoblast cell of the early emrbyo divides to form mesodermal blocks between the ectoderm and the endoderm and replaces the blastocoel. The split that appears in each mesodermal block leads to the formation of Schizocoelom.
II. Enterocoelomates : Animals in which the body cavity is formed from the mesodermal pouches of archenteron are called enterocoelomates: Echinoderms, hemichordates and chordates are the enterocoelomates. In these animals, mesodermal pouches that evaginate from the wall of the archenteron into the blastocoel are fused with one another to form the enterocoelom. All the enterocoelomates are deuterostomes and they show radial and indeterminate cleavage.
