Difference Between Protostomes and Deuterostomes

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Protostomes and Deuterostomes

The animal kingdom is diverse in terms of morphology, anatomy, and physiology. Despite the differences, one thing that remains common among most animals is that they all start life as a single cell. As the organism attains maturity, it develops some unique characteristics that set it apart from others and help in the systematic classification of animals into different groups. One such group of animals are called the metazoans whose body cells are differentiated into tissues and organs. Based on the embryonic development of these metazoans, they are classified into two groups - protostomia and deuterostomia (or protostomes and deuterostomes).


Protostomes

The word protostome is derived from the Greek words “proto” meaning first, and “stoma”, which means opening or mouth. In literal terms, protostome means ‘first mouth’. The reason for this naming is based on their mode of embryonic development. When the embryo develops, the protostomes first form the mouth from the blastopore, and the anus develops later after the mouth has been created.

A protostome is a group of animals that includes invertebrates like arthropods (insects and crabs), molluscs (clams and snails), and worms of phylum Annelida. Developments in the last few decades in molecular biology techniques such as DNA sequence analysis has suggested that protostomes can be divided into two major super phyla or groups called Spiralia and Ecdysozoa. While Spiralia includes Platyhelminthes, molluscs, and annelids, Ecdysozoa comprises nematodes and arthropods.


Deuterostomes

The word deuterostome also has a Greek origin; “deutero” means second, and “stoma” means mouth or opening. Thus, deuterostome translates to ‘second mouth’. In deuterostomes, the opening at the bottom of the gastrula, known as the blastopore, forms the anus. The mouth is formed at later stages of a deuterostome’s embryonic development. In deuterostomes, the anus and the mouth are located at the opposite ends of the blastopore, and a digestive tract develops in the middle to connect the mouth and the anus.

Deuterostomes further have three significant classifications which are - Chordata, Echinochordata, and Hemichordata. Chordata includes all the vertebrates like birds and mammals. So, are humans deuterostomes? Well, yes, they are!

Echinochordata and Hemichordata comprise invertebrates. While Echinochordata includes starfish, sea urchins, and sea cucumbers, Hemichordata contains grapholites and acorn worms.

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Difference Between Protostomes and Deuterostomes

The most significant difference between protostomes and deuterostomes arises due to the dissimilarities in their embryonic development. In protostomes, the blastopore forms in the mouth, but in deuterostomes, the blastopore is involved in the development of the anus. 

Now let us go through some other striking differences between protostomes and deuterostomes:

  • Origin of Mouth and Anus: In protostomes, the origin of the anus is secondary, but in deuterostomes, the formation of the mouth is secondary.

  • Origin of Mesoderm: The archenteron is the rudimentary alimentary canal formed during the early stages of embryonic development that later forms the mesoderm and endoderm. The development of archenteron is not seen in protostomes, but, for deuterostomes, the primitive gut formation takes place in early embryos.

  • Late Gut Development: In protostomes, the anus is formed by tunnelling of the gut into the embryo. But in deuterostomes, the formation of the mouth is by the gut tunnelling into the embryo.

  • Development of Coelom: In protostomes, the body cavity or the coelom is formed by splitting of the mesodermal layer. Thus, they are schizocoelomates since the coelom is created by schizocoely. On the other hand, the deuterostomes are enterocoelomates where the body cavity or the coelom is formed by the mesoderm pinching off from the gut.

  • Type of Cleavage: In protostomes, the kind of cleavage is determinate, meaning that the blastomere of the early embryonic stage is incapable of developing into independent embryos. But deuterostomes undergo indeterminate division whereby the early blastomeres can develop into complete embryos.

  • Nervous System: Protostomes have a solid, ventral nerve cord. In deuterostomes, the nerve cord is hollow, with some possessing pharyngeal gill slits.

  • Body Complexity: Deuterostomes have a more evolved and complex body organisation than the protostomes.

  • Type of Cell Ciliation: The cells in protostomes are multi-ciliated, but those in deuterostomes are mono-ciliated.

  • Number of Phyla: Protostomes include more species and phyla compared to deuterostomes.

  • Classification: Protostomes can be grouped into Spiralia and Ecdysozoa. The groups under deuterostomia include Echinoderms, Chordates, Hemichordates, and other higher and more complex organisms like human beings.

FAQ (Frequently Asked Questions)

1. What do the Terms Gastrulation, Archenteron, and Blastopore Mean?

  • In most animals, gastrulation is an early phase of embryonic development when the simple spherical ball of cells called the blastula undergoes extensive cell movements and reorganisation to develop into a multi-layered structure called the gastrula. During gastrulation, the three primary germ layers, that is, the ectoderm, endoderm, and the mesoderm are formed and organised into their appropriate locations.

  • When a developing embryo undergoes gastrulation, a primary gut or digestive tube is formed which is known as the archenteron. The archenteron later develops into the endoderm and mesoderm.

  • Blastopore is an opening of the gastrula through which the latter communicates with the outside. The blastopore develops into the mouth or the anus during maturation of most animals. In others, the blastopore serves as the canal joining the neural tube (primitive nervous system) to the primitive gut. 

2. What are Diploblastic and Triploblastic Animals?

Diploblastic and triploblastic are two different stages in blastula formation. The germ layer is the primary layer of cells formed during embryogenesis. In vertebrates, gastrulation usually gives rise to three germ layers: endoderm, mesoderm, and ectoderm. Eumetazoans or animals with a more complex structure than sponges produce two to three germ layers. Diploblastic animals produce only two germ layers - endoderm and ectoderm (example: radially symmetrical animals like starfish). Triploblastic animals produce all the three germ layers, that is, ectoderm, endoderm, and mesoderm. Animals that are bilaterally symmetrical, such as humans, are triploblastic. Hence, the key difference between diploblastic and triploblastic animals is that diploblasts produce two germ layers (excluding the mesoderm) and triploblasts produce all the three germ layers.