Difference Between Diploblastic and Triploblastic

The main difference between diploblastic and triploblastic animals is that diploblastic animals produce two germ layers excluding the mesoderm, while triploblastic animals produce all three germ layers.

Diploblastic and triploblastic refer to two different types of blastula stages. The primary layer of cells formed during embryogenesis is referred to as the germ layer. In vertebrates, three germ layers can be generally identified in the gastrula: endoderm, mesoderm, and ectoderm. Animals with a more complex structure than a sponge (eumetazoans), however, produce two or three germ layers. Radially symmetric animals are diploblastic. They produce only two germ layers: the endoderm and the ectoderm. Bilaterally symmetric animals are triploblastic. They produce the three germ layers: endoderm, ectoderm, and mesoderm. 

Key Areas Covered

1. What is Diploblastic
     – Definition, Features, Examples
2. What is Triploblastic
     – Definition, Features, Examples
3. Difference Between Diploblastic and Triploblastic
     – Comparison of Key Differences

Key Terms

Diploblastic, Triploblastic

What is Diploblastic

During gastrulation, diploblastic organisms form a gastrula which consists of two primary germ layers. These two germ layers are composed of an endoderm and an ectoderm, but not a mesoderm. Endoderm gives rise to true tissues combined with the gut. On the other hand, ectoderm gives rise to epidermis, nervous tissue, and nephridia. Since diploblastic animals are lacking a mesoderm, they cannot generate body cavities. However, a non-living layer exists between the endoderm and the ectoderm. This layer is often gelatinous and is referred to as mesoglea. Mesoglea helps to protect the body and lines the gut.

Figure 1: Diploblastic Animal’s Gastrula

Diploblastic animals possess radial symmetry. Cnidaria and Ctenophora are considered as diploblastic. Jellyfish, comb jellies, corals and sea anemones are examples of diploblastic animals.

What is Triploblastic

Triploblastic organisms form all three primary germ layers – endoderm, ectoderm and mesoderm – during the gastrulation of the blastula. Mesoderm development is a characteristic feature of triploblastic animals. Mesodermal cells differentiate through the interactions of both ectodermal and endodermal cells. Coelom is developed from the mesoderm. Inside the coelom, freely moving organs are formed, providing protection against shocks by fluid cushions. These organs can grow and develop without the aid of the body wall. Mesoderm forms muscle, bone, connective tissues, circulatory system, notochord, etc. Other than that, endoderm develops into the lungs, stomach, colon, liver, urinary bladder, etc. Ectoderm develops into epidermis, hair, eye lens, brain, spinal cord, etc.

Figure 2: Differentiation of Three Germ Layers

All animals, from flatworms to humans, are triploblastic. They belong to the clade: Bilateria and possess bilateral symmetry. Triploblastic animals are further divided into sections such as acoelomates, eucoelomates and pseudocoelomates. Acoelomates lack a coelom, whereas eucoelomates consist of a true coelom. Pseudocoelomates are composed of a false coelom. Eucoelomates can be again divided into two sections: protostomes and deuterostomes. Protostomes develop the mouth from the blastopore, whereas the deuterostomes develop the anal opening from the blastopore. It is believed that diploblastic animals gave rise to triploblastic animals around 580 to 650 million years ago.

Difference Between Diploblastic and Triploblastic

Definition

Diploblastic animals produce two primary germ layers, endoderm and ectoderm, during gastrulation, while triploblastic animals produce three primary germ layers: endoderm, ectoderm, and mesoderm.

Biological Symmetry

While diploblastic animals are radially symmetric, triploblastic animals are bilaterally symmetric.

Mesoderm Development

Diploblastic animals do not have a mesoderm. In between the endoderm and the ectoderm, mesoglea can be identified. However, triploblastic animals develop a mesoderm.

Body Cavities

Moreover, diploblastic animals do not have body cavities. Most triploblastic animals develop a body cavity, the coelom.

Endoderm Development

The endoderm of diploblastic animals forms true tissues and the gut, while the endoderm of triploblastic animals forms lungs, stomach, colon, liver, urinary bladder, etc.

Ectoderm Development

The ectoderm of the diploblastic animals forms the epidermis, nervous tissue, and nephridia, while the ectoderm of the triploblastic animals forms the epidermis, hair, eye lens, brain, spinal cord, etc.

Development of Organs

Diploblastic animals do not have organs, whereas triploblastic animals have true organs such as hearts, kidneys, and lungs.

Complexity of the Organism

Moreover, diploblastic animals are not complex, while triploblastic animals are much more complex than diploblastic animals.

Examples

Jellyfish, comb jellies, corals and sea anemones are examples of diploblastic organisms, while molluscs, worms, arthropods, echinodermata and vertebrates are examples of triploblastic organisms.

Conclusion

Animals like sea sponges exhibit the simplest organisation, consisting of only a single germ layer. Though they are composed of differentiated cells, they lack true tissue coordination. Diploblastic animals, on the other hand, exhibit an increased complexity than sponges, containing two germ layers, the ectoderm and endoderm. They are organised into recognisable tissues. However, triploblastic animals possess an additional germ layer, the mesoderm, which can develop complex organs in the body. Thus, the key difference between diploblastic and triploblastic animals is the type of cleavage during embryonic development.

Reference:

1. “Germ layer”. Wikipedia, the free encyclopedia, 2017. Accessed 17 Feb 2017
2. Myers P. Z. “Diploblasts and triploblasts”. Pharyngula, ScienceBlogs. 2006. Accessed 17 Feb 2017

Image Courtesy:

1. “Blastula” By Abigail Pyne – Own work (PD-self) via Commons Wikimedia
2. “Location of the intermediate mesoderm nephrogenic cord” By Davidson, A.J. – Davidson, A.J., Mouse kidney development (January 15, 2009), StemBook, ed. The Stem Cell Research Community, StemBook, doi/10.3824/stembook.1.34.1 (CC-BY-3.0) via Commons Wikimedia