Difference Between Protostomes and Deuterostomes

Main Difference – Protostomes vs Deuterostomes

Protostomes and Deuterostomes are two clades of animals that make up the Bilateria, a clade of subkingdom: Eumetazoa, which consists of animals composed of bilateral symmetry and three germ layers. The main difference between protostomes and deuterostomes originates from the development of their blastopore, a transient opening of the archenteron to the exterior of the embryo at the gastrula stage, which is related to the later development of mouth or anal openings.  The main difference between protostomes and deuterostomes is that in protostomes, the blastopore is developed into the mouth whereas in deuterostomes, the blastopore is developed into the anal opening. Both Protostomia and Deuterostomia forms the clade: Nephrozoa.   

This article looks at,

1. What are Protostomes
      – Structure, Characteristics, Examples
2. What are Deuterostomes
      – Structure, Characteristics, Examples
3. What is the difference between Protostomes and Deuterostomes

What are Protostomes

Protostomes belong to the clade Nephrozoa of Bilateria. In protostomy, the blastopore forms the mouth of the protostomes. During the development of the gut, protostomes first develop the mouth from the dent, blastopore. The anal opening is formed secondly by the tunneling of the gut through the embryo. Some bilaterian animal forms such as Acoelomorpha, consist of a single mouth with no anus. A solid mass of the embryonic mesoderm of protostomes splits to form a coelom. Thus, they are called schizocoelomates. The coelom cannot be identified in some protostomes such as Priapulids. Some phyla of protostomes exhibit spiral cleavage in the absence of large yolk concentration. Equal spiral cleavage is the most occurring and sometimes they undergo unequal cleavage. The developmental fate of the cells is determined during the early development of embryo. Thus, protostomes belong to the determinate cleavage. A solid, ventral nerve cord is found in protostomes.

Protostomes can be further divided into two superphyla: Ecdysozoa and Lophotrochozoa. Examples for Ecdysozoa are arthropods and nematodes whereas molluscs, annelids, platyhelminths and rotifers are the Lophotrochozoas. 

Figure 1: Fate of the Blastopore

What are Deuterostomes

Deuterostomes belong to the subtaxon: Deuterostomia of Bilateria. In deuterostomes, the original blastopore develops into the anal opening. Eventually, the gut is tunneled to form the mouth. The early development of the gut is referred to as the archenteron in deuterostomes. Longitudinal pouches of the archenteron forms the coelom of the deuterostomes. Thus, they are called enterocoelous. The coelom later develops into separated cavities. In the absence of large yolk concentration, deuterostomes exhibit radial cleavage. They have either parallel spindle axes to the polar axis of the oocyte or right angles to the polar axis. Their cytoarchitectural features are undisturbed. Therefore, deuterostomes belong to the indeterminate cleavage. Three major clades of deuterostomia is identified: Hemichordata, Echinodermata and Chordata. Deuterostomes consist of a highly modified nervous system. Crordata is containing a hollow nerve code. A tubular nerve code is contained by Hemichordatas. Deuterostomes are characterized by a hollow nerve cord, pharyngeal gill slits and circular and longitudinal muscles with a segmented body. Examples for Hemichordata are graptolites and acorn worms. Starfish, sea urchins and sea cucumbers are the examples of Echinodermata. Chordata are all vertebrates and their kin.

Figure 2: Examples of Deuterostomes

Difference Between Protostomes and Deuterostomes

Development of the Blastopore

Protostomes: The blastopore develops into the mouth.

Deuterostomes: The bastopore develops into the anal opening.

Origin of Mesoderm

Protostomes: No archenteron development is found in protostomes.

Deuterostomes: The early development of the gut is called archenteron in deuterostomes.

Later Development of The Gut

Protostomes: The gut is tunneled into the embryo to form the anus.

Deuterostomes: The gut is tunneled into the embryo to form the mouth.

Development of Coelom

Protostomes: Protostomes are called schizocoelomates, since the coelom is developed by splitting of the solid mass of the embryonic mesoderm.

Deuterostomes: Deuterostomes are called enterocoelous since the longitudinal pouches of the archenteron forms the coelom.

Type of Cleavage

Protostomes: Protostomes exhibit determinate cleavage.

Deuterostomes: Deuterostomes exhibit indeterminate cleavage.

Type of the Holoblastic Cleavage

Protostomes: Protostomes undergo spiral cleavage.

Deuterostomes: Deuterostomes undergo radial cleavage. 

Nervous System

Protostomes: Protostomes are composed of a solid, ventral nerve cord.

Deuterostomes: Deuterostorms are composed of a hollow nerve cord and pharyngeal gill slits.

Conclusion

Protostomes develops mouth from the blastopore during the development of embryo. On the other hand, deuterostomes develop anus from the blastopore. Both protostomia and deuterostomia also consist of differences in the development of their three germ layers. Deuterostomes bear a higher modified nervous and muscular systems. The key difference between protostomes and deuterostomes is thus the fate of blastopore in their embryonic development. 

Reference:
1.Martín-Durán, José M., Yale J. Passamaneck, Mark Q. Martindale, and Andreas Hejnol. “The developmental basis for the recurrent evolution of deuterostomy and protostomy.” Nature News. Nature Publishing Group, 05 Dec. 2016. Web. 16 Feb. 2017.
2.“Embryological origins of the mouth and anus”. Wikipedia, the free encyclopedia. 2017 . Accessed 16 Feb. 2017
3.“Cleavage (embryo)”. Wikipedia, the free encyclopedia. 2017 . Accessed 16 Feb. 2017
4.“Protostome”. Wikipedia, the free encyclopedia. 2017 https://en.wikipedia.org/wiki/Protostome. Accessed 16 Feb. 2017
5.“Deuterostome”. Wikipedia, the free encyclopedia. 2017 . Accessed 16 Feb. 2017

Image Courtesy:
1.“Protovsdeuterostomes”. By YassineMrabet – Own work (CC-BY-SA-3.0) via Commons Wikimedia
2.“Reef2589”. By Larry Zetwoch, Florida Keys National Marine Sanctuary –  (Public Domain) via Commons Wikimedia