Types of Coelom

Pseudocoelom :-

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 is any invertebrate animal with a three-layered body and a pseudocoel. Nematoda (roundworms), Rotifera (rotifers), Kinorhyncha, Nematomorpha, Nematomorphs or horsehair worms, Gastrotricha, Loricifera, Priapulida, Acanthocephala, spiny-headed worms, Aschelminth animals, Entoprocta are the examples of Pseudocoelomates. The coelom was apparently lost of 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 usd as a descriptive term.

Important characteristics as the psudocoelomates are adated to survive that they lack a vascular blood system, as a diffusion and osmosis circulate nutrients and waste products throughout the body. They lack a skeleton hydrostatic pressure gives the body a supportive framework that acts as a skeleton. Furthermore they have no segmentation. Their body wall comprises of epidermis and muscle often syncytial usually covered by a secreted cuticle. They are mostly microscopic and chosen a parasite mode life (although some are free living ).

Acoelomate :-   

Lacking a fluid filled body cavity presents some serious disadvantage. Fluids do not compress, 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.

Organisms showing acoelomates formation include the platyhelminthes (flatworms, tapeworms, etc), the Cniderians (jellyfish and allies), and the Ctenophores (comb jellies). The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorsoventral flattening.

The coelom is defined as a body cavity or space than runs the length of a vertebrate. It separates the body into an inner tube and an outer tube. the coelom forms when the lateral plate mesoderm splits.

Evolution of Coelomate Animals :-  

In the Cniderian, the space between the ectoderm and endoderm tissue layers is filled with an acellular mesoglea. In the platyhelminth (flatworms) these mesoderm fills the space between mesoderm and the endoderm :

The advantage of the flatworm body plan is the superior pull muscles can make using solid mesoderm as a lever, versus gel-like mesoglea. But, body organs can not move freely but are embedded in solid mesoderm tissue. It is not more difficult for materials to move from the gut to the body wall. Probably to solve the problems the is to have some sort of body cavity, called a coelom, in which the body organs lie bathed in body fluid. Movement of the body wall would not squeeze the organs, organs could glow without being pressed by the body wall and diffusion of nutrients and wastes would be easier.