The advantage of the flatworm body plan, is the superior pull muscles can make using solid mesoderm as a lever, versus gel-like mesoglea.
1. Body organs cannot move freely but are embedded in solid mesoderm tissue.A solution to both problems 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 grow without being pressed by the body wall, and diffusion of nutrients and wastes would be easier.
2. It is more difficult for materials to move from the gut to the body wall.
1. The hydrostatic skeleton - a fluid-filled body cavity provids a more rigid structure than mesoderm for muscles to pull against and thus be a more efficient skeletal support system.II. Result - Faster locomotion and Larger body size possible.
2. Circulatory System
Absorbed nutrients can be circulated around the body and metabolic wastes can be carried to the body surface more efficiently by fluid than by solid mesoderm.
Two types of Body Cavities:
In addition to having a body cavity, organisms with pseudocoeloms also have a
complete digestive tract -- separate openings for food to enter and undigested
material to leave. This makes digestion and feeding more efficient because the
animal can eat before it has finished digesting its previous meal.
Organisms that have pseudocoeloms are not a monophyletic group (in other words, the pseudocoelom is a homoplasious character that occured more than once).
Psuedocoelomates contain a variety of small oganisms, including
Nematodes (roundworms):Two kind of True Coeloms
A true coelom is lined on all sides by mesoderm which gives rise to muscles that surround the gut as well as underlying the body wall. This allows more efficient digestion because food can be pushed through the digestive tract by muscles.
Schizocoeloms are found in animals known as PROTOSTOMATES
Enterocoeloms are found in animals known as DEUTEROSTOMATES Schizocoelom:
The body cavity is formed by schizocoely - Schizocoely means that the body cavity is formed from blocks of mesoderm around the gut that enlarge and hollow out. The end result is a pair of coelomic cavities.
Cleavage is determinate - This means that the fate of each individual cell is already established when it is formed. Each cell can only develop into a specific cell type.
Cleavage is radial - Each cell division is equal and results in cells of all the same size.This can be seen in the diagram of radial cleavage shown below:
Cleavage is indeterminate.This means that the fate of each individual cell is not
determined when it is formed and each cell can form one of many cell types.