to any developed digestive tract. Some organisms may not possess a coelom or may have a false coelom (pseudocoelom). Animals having coeloms are called coelomates, and those without are called acoelomates. There are also subtypes of coelom: schizocoelom: develops from split in mesoderm found in annelids, arthropods and molluscs haemocoelom: true coelom reduced and cavity filled with blood found from arthropoda to mollusca enterocoelom: develops from wall of embryonic gut found from echinodermata to chordata Coelomate phyla According to Brusca and Brusca, the following bilaterian phyla possess a coelom: Nemertea, traditionally viewed as acoelomates Priapulida Annelida Onychophora Tardigrada Arthropoda Mollusca Phoronida Ectoprocta Brachiopoda Echinodermata Chaetognatha Hemichordata Chordata Pseudocoelomates In some protostomes, the embryonic blastocoele persists as a body cavity. These protostomes have a fluid filled main body cavity unlined or partially lined with tissue derived from mesoderm. This fluid-filled space surrounding the internal organs serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton. A pseudocoelomate or blastocoelomate is any invertebrate animal with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates. "Pseudocoelomate" is no longer considered a valid taxonomic group, since it is not monophyletic. However, it is still used as a descriptive term. Important characteristics: lack a vascular blood system diffusion and osmosis circulate nutrients and waste products throughout the body. lack a skeleton hydrostatic pressure gives the body a supportive framework that acts as a skeleton. no segmentation body wall epidermis and muscle often syncytial usually covered by a secreted cuticle most are microscopic parasites of almost every form of life (although some are free living) eutely in some loss of larval stage in some possibly pedomorphism Pseudocoelomate phyla According to Brusca and Brusca, bilaterian pseudocoelomate phyla include: Rotifera Kinorhyncha Nematoda Nematomorpha Acanthocephala Loricifera Some authors list the following phyla as pseudocoelomates: Ecdysozoan pseudocoelomates Nematoda (roundworms) Nematomorpha (nematomorphs or horsehair worms) Loricifera Priapulida Kinorhyncha Spiralian pseudocoelomates Gastrotricha Entoprocta Rotifera (rotifers) Acanthocephala (spiny-headed worms) Acoelomates Acoelomates lack a fluid-filled body cavity between the body wall and digestive tract. This can cause some serious disadvantages. Fluid compression is negligible, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal’s outer surface. The coelom