Evolution of Coelomate Animals

In the cnidarians, the space between the ectoderm and endoderm tissue layers is filled with an acellular mesoglea. In the platyhelminthese (flatworms) 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.


1. Body organs cannot move freely but are embedded in solid mesoderm tissue.

2. It is more difficult for materials to move from the gut to the body wall.

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.

Advantages of a Coelom
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.

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.

II. Result - Faster locomotion and Larger body size possible.

Two types of Body Cavities:
True Coelom


The pseudocoelm is a blastocoel (the space in the blastula) that is retained to adulthood.

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):
and, Kinorhynchs

Two kind of True Coeloms
1. Schizocoelom
2. Enterocoelom

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.

True Coelom

Schizocoeloms are found in animals known as PROTOSTOMATES
Enterocoeloms are found in animals known as DEUTEROSTOMATES Schizocoelom:


Cleavage is spiral. In spiral cleavage the first two divisions of the egg are equal, but the rest are unequal.
This results in some small and some large cells, as can be seen in the diagram below:

The blastopore forms the mouth.

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:

The blastopore doesn't form the mouth, it usually forms the anus.

The body cavity is formed by enterocoely. Enterocoely is the formation of the body cavity by outpocketings of the primitive gut that break off and form the coelom.

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.