There is a major group of animals which are called bilaterally symmetrical animals due to the very fact of their bilateral body symmetry. The bilateral symmetry refers to an animal’s body’s such a shape that a plane passing through the body divides it into two halves that are mirror images of each other.
The bilateral animals can be divided into two major groups named protostomes and deuterostomes referring to the respective events of protostomy and deuterostomy happing in the early developmental phases of their lives.
A clade of animals undergoing protostomy during embryonic development is called as protostomes. The word clade refers to a lineage of an animal and all its ancestors. For example, the main species and all of its descendent species can be considered a clade. A condition in which animals are characterized by the conversion of the first opening of the blastomere into a moth in the complete individual.
There is another clade of animals in which developmental events follow deuterostomy. It closely belongs to the protostome class with the exception that the first opening of the blastomere becomes the anus in the final organismic body.
Tabular comparison between Protostomes v/s Deuterostomes
|Basis of Difference||Protostomes||Deuterostomes|
|Pattern of cleavage||Spiral cleavage||Radial cleavage|
|Blastosphere’s first opening||Forms mouth||Forms anus|
|Type of cleavage||Determinate / mosaic cleavage||Indeterminate / regulative cleavage|
|Presence of archenteron||No archenteron||Archenteron develops in early stages|
|Nervous system||Solid and ventral / lateral nerve cord||Hollow nerve cord and pharyngeal gill slits, dorsal location|
|Origin of mesoderm||Separate germ layer||archenteron|
Differences between protostomes and deuterostomes
The differences between the protostomes and deuterostomes arise as the development of the zygote into a complete organism takes place. This starts early in the development after the gametes i.e., a sperm and an egg, fuse to form a zygote which then undergoes further developmental events.
During the very initial events, the zygote is divided in such a way so as to form two poles. The top pole is known as ‘animal pole’ while the bottom one is called as ‘vegetal pole’.
After the formation of poles, further divisions happen in a process named by developmental scientists as ‘cleavage’. This process results in the formation of a dense ball like mass of cells and this ball like structure has been named as ‘blastosphere’ and the cells are known as blastomeres.
Now let us take a deep look into how the events of development lead to these two classes of organisms and what are the key differences between these developmental events leading to the final body forms of protostomes and deuterostomes.
One of the most fundamental differences between protostome and deuterostome developments lies in the pattern of cleavage which results in the formation of blastosphere.
In protostomes, the zygote follows a spiral cleavage pattern. It is characterized by the presence of one tier of the blastomeres over the other in such a way that the upper-tier lies at the cell junctions of the next tier which is present on its lower side. In this way, the successive blastomeres from a spiraling structure from one pole to the other.
In deuterostomes, the form of cleavage leading to blastosphere formation is called as radial cleavage. It is characterized by the presence of the one tier of the cells of the blastomeres directly over the preceding tier of the cells.
Both of these cleavages are also illustrated in the following figure.
Origin of Mouth and Anus
The origin of mouth and anus during the embryological development of bilaterally symmetrical animals is another important feature creating a distinction between the two classes i.e., protostomes and deuterostomes. We have already discussed the formation of the blastomere, now let us take a look at what happens next.
As the blastosphere undergoes further developments, the next major event is the formation of an indentation on its one side known as the blastopore. The next difference between protostomes and deuterostomes lies in the final organ being formed by this blastopore.
In protostomes, the blastopore forms the mouth while the anus is formed by on the side opposite to the blastopore. Contrastingly, vice versa happens in deuterostome i.e., the anus is formed at the blastopore and mouth opposite to the blastopore.
The two classes also differ with respect to the types of cleavage of the blastomeres when forming blastosphere. This cleavage has two basic types: determinate / mosaic cleavage (in protostomes) and indeterminate / regulative cleavage (in deuterostomes).
Protostomes are characterized by the presence of determinate cleavage during the formation of the blastosphere. In determinate cleavage, also known as mosaic cleavage, the developmental fate of the cells is already determined during the early developmental events of the embryo.
Contrarily, deuterostomes follow indeterminate type of blastomere’s cleavage. As the name indicates, in this type of cleavage the fate of the individual cells is not set beforehand. In fact, each cell of the blastosphere has an equal potency to be developed into any type of cell of an individual. This condition is also known as pluripotency of the cells.
Archenteron refers to the presence of a rudimentary alimentary canal which is formed during early development of the embryo, more precisely at the gastrula stage. This holds the basis of another major difference between the two sets of bilateral animals i.e., protostomes and deuterostomes.
There is no archenteron development in the protostomes, on the other hand, archenteron forms in the early stages of the deuterostome embryonic development which is usually developed into mesoderm and endoderm of the living organism later on.
In both the clades, the nervous system represents a portion of the ectoderm which is internalized and performs the functions of receiving and integrating the information received form the sensory system. The presence of different types of nervous systems is also a distinguishing feature between protostomes and deuterostomes.
The protostomes contain a solid nervous system located on either the ventral or lateral side of the body in the form of a solid nerve cord.
On the other hand, the nervous system in the deuterostomes are dorsally located in the form of a hollow nerve cord. Moreover, they are also characterized by the presence of the pharyngeal gill slits.
In all bilateral animals, there are three primary germ layers which are formed in the early embryo. One of these is mesoderm which is the middle layer and the other two are ecto- and endoderm (outer and inner layers, respectively).
In deuterostomes, the mesoderm is derived from the archenteron wall which is an early digestive tract in turn derived from the endoderm of the early embryo. Whereas in protostomes, the mesoderm is formed along with ecto- and endoderm as a separate layer from which a hollow body cavity known as coelom is derived.