Deuterostomes

In Deuterostomia, a significant group of metazoans, embryonic development is characterized by the formation of the anal opening from the blastopore, marking the posterior end of the body. Subsequently, the mouth forms later in development. This unique sequence, with the anus preceding the mouth in formation, is the defining feature placing these organisms within Deuterostomia, often termed "second mouth." 

1.0Deuterostomes Definition

Deuterostomes, alongside Protostomes, represent a significant division within the animal kingdom. Their distinguishing characteristics include radial cleavage of the blastula, a developmental process where cell fate remains flexible until later stages, and the coelom formation through a process known as enterocoely, where the embryonic gut forms pouches. The three primary phyla within Deuterostomes are Hemichordata, Echinodermata, and Chordata.

Deuterostomes include organisms from the phyla Echinodermata and Chordata, such as starfish, sea squirts, sea urchins, and lancelets.

2.0Characteristics of Deuterostomes

The developmental characteristics of the Deuterostome are as follow:

• Pattern of embryonic cleavage- Embryonic cleavage follows a radial pattern, with division planes aligned either parallel or perpendicular to the polar axis. Blastomeres are positioned in a stacked formation, directly above or below each other.
  
• Process of embryonic blastomeres- Cleavage exhibits indeterminate, meaning that if blastomeres are separated at the 4-cell stage, each has the potential to develop into a complete organism. This process is regarded as regulative because individual blastomeres, if isolated, can regulate their developmental pathway. If one cell is depleted, the remaining cell's descendants can compensate by generating structures in the embryo that would have originated from the lost cell. Such dynamic developmental flexibility characterizes regulative development.
• Process of blastopore- The blastopore transforms into the mature anus, followed by the emergence of the mouth from a secondary opening on the dorsal surface of the embryo.
• Formation of mesoderm and coelom - Mesodermal tissue originates from the endodermal wall of the archenteron (gastrocoel) through an outgrowth process. The coelom is created by the evagination of pouches from the archenteron wall (gastrocoel) , with each pouch eventually detaching and developing into a distinct coelomic cavity. This developmental mechanism is known as enterocoely, resulting in the formation of an enterocoelom.

3.0Classification of Deuterostomes

Below is an extensive classification of deuterostomes, including various subphyla. This classification serves as a valuable reference for understanding the evolutionary origins and characteristics of deuterostomes.

Superphylum Deuterostomia 

4.0Protostomia Definition

Protostomia encompasses a group of metazoans where the initial formation of the mouth occurs from the blastopore located at the anterior end. Subsequently, the anus develops to complete the alimentary canal. This classification is based on the precedence of mouth formation, leading to inclusion in the 'Protostomia' division of the animal kingdom. Additionally, protostomes typically exhibit a ventral nerve cord arrangement.

5.0Characteristics of Protostomia

The developmental characteristics of the Protostomia are as follows:

• Pattern of embryonic cleavage - In protostomes, cleavage is characterized by a spiral pattern, wherein the axis of the cleavage plane is oblique. This results in blastomeres arranging themselves spirally, with one tier of cells alternating with the next. The spiral cleavage pattern becomes apparent around the 64-cell stage, particularly evident by the 6th cleavage.
• Process of embryonic blastomeres -  Fate of blastomeres is determined very early during holoblastic cleavage. This is called determinate or mosaic cleavage, which means blastomeres are destined to form a particular organ in the very early stage of cleavage.
• Process of blastopore - In certain organisms, such as Mollusca, the blastopore transforms into the mouth during development.  In some mollusks, polychaetes, and onychophorans, the blastopore can develop into both the mouth and anus in the adult stage.
• Formation of mesoderm and coelom - The mesoderm is initiated from the fourth cell, referred to as the mesentoblast or '4d' cell, which proliferates to increase in number. The coelom forms through the division of the mesodermal cell mass, a process termed schizocoely, resulting in the development of a schizocoelom.

6.0Difference Between Protostomia and Deuterostome