Deuterostomes vs Protostomes 

Major differences between deuterostomes and protostomes 

Coelomate animals are distinguished according to the fate of blastopore into two groups the Protostomes and the Deuterostomes. In group protostome,( proto - first; stoma - mouth) the blastopore eventually becomes the mouth of the adult, whereas in the group deuterostome( deutero - second; stoma - mouth), the blastopore develops into an anus and a second opening which develops later forms the mouth.

 Out of the major coelomate phyla,  annelida, mollusca and arthropoda are protostomes whereas echinodermata and chordata are deuterostomes.

Deuterostomes vs Protostomes diagram

( The diagram shows the early developmental stages of deuterostomes and protostomes,  depicting the major difference. )

The major points of differences are:

1. Cleavage 

Deuterostomes :

Deuterostomes showcase radial cleavage in which the anus is the first opening of the embryo.

In radial cleavage, the mitotic spindle remains oriented vertically (without tilting), starting from the third cleavage, as a result, the blastomere lies exactly one above the other ( i.e. The lower four blastomere is exactly below the upper four blastomere ). As the cleavage continues, the new tier of cells lies directly below the previous ones.

Protostomes:

Protostomes typically show spiral cleavage.

This spiral effect begins with the onset of the division of the embryonic cells from four to eight cells.

In spiral cleavage the mitotic spindle is obliquely tilted, resulting in a condition where the two tiers of four blastomeres are not exactly one above the other. 

This staggered arrangement of tiers of cells continues as cells are added, giving rise to the spiral arrangement of blastomeres.

( A-D ) - Radial and spiral cleavage in eggs with very little yolk, ( A-C ) - radial cleavage, embryo viewed from the side, ( D ) embryo viewed from the top, ( E, G ) spiral cleavage, embryo viewed from the top, ( F ) viewed from the side.

2. Developmental regulations :

Deuterostomes :

 The fate of the blastomere is not fixed irreversibly, i.e. Indeterminate type of developmental regulation which can result in monozygotic twinning ( each cell at the two-cell stage can develop into a complete organism ).

Protostomes :

Showing spiral cleavage. The fate of the blastomere is determinate, from the onset of cleavage, so no monozygotic twinning.

3. Blastopore :

Deuterostomes :

The site of the anus marks the posterior end of the longitudinal body axis at the gastrula stage and the mouth opens on the opposite end of this axis.

Protostomes :

The anterior pole of the body axis is marked by the site of the mouth.

4. Mesoderm :

Deuterostomes :

The mesoderm derives from the endoderm.
Here the mesoderm develops from the roof  of the primitive gut i.e. the archenteron, and then sheets out in both directions between the ectoderm and endoderm, until it meets mid-ventrally, completely seperating the other two germ layers.

Protostomes :

The Mesoderm does not form from the endoderm of the archenteron but develops from a single blastomere which is distinguishable in early cleavage.

Here a single cell identifiable in early cleavage is the source of all Mesoderm. After gastrulation, this cell which is now located near the blastopore, begins to proliferate forming mass of cells filling the cavity between ectoderm and endoderm, forming the mesoderm.

 

5. Body cavity:

Deuterostomes :

The true coelom is always present. The development is characteristically enterocoelous i.e.,

• The Mesoderm side pushes outward and expands into a pouch-like coelomic compartment.
• This then pinches off and forms a mesoderm-bound space surrounding the gut.
• Example: Sea stars, fish, frogs etc.

Protostomes :

If present, may be either pseudocoel or a true coelom with schizocoelous development, i.e. arises as a cleft in a formally solid block of mesodermal tissue. The coelom forms from,

• Endodermal cells, move to blastopore and develop into mesoderm.
• Mesoderm separates or splits to form a cavity (coelom).
• Example: Earthworm, snails etc.

6. Muscle contraction :

Deuterostomes :

The high-energy storage compound used in muscle contractions is creatine phosphate.

Protostomes :

In the case of protostomes, the high-energy storage compound used in muscle contractions is arginine phosphate.

7. Nerve cord :

Deuterostomes :

The major longitudinal nerve cord, if present, is hollow and lies dorsal to the gut.

Protostomes:

The major longitudinal nerve cord, if present, is solid and lies ventral to the gut.

8. True heart :

Deuterostomes :

If present, usually lies ventral to the gut.

Protostomes :

If present, lies dorsal to the gut.