Problem 255 - Objective Biology for NEET

Question

Assertion: Coelenterates are triploblastic. Reason: Coelenterates contain mesoderm in between ectoderm and endoderm.

Step-by-Step Solution

Answer

Both the assertion and the reason are incorrect. Coelenterates are not triploblastic; they are diploblastic. They do not have a mesoderm layer in between their endoderm and ectoderm layers.

Step 1: Define the Terms

In this step, Coelenterates are defined as a group of marine animals that include sea anemones, corals, and jellyfish. 'Triploblastic' refers to creatures that develop from three germ layers in embryonic development: ectoderm, mesoderm, and endoderm. Each term (ectoderm, mesoderm, and endoderm) refers to one of these germ layers.

Step 2: Analyze the Assertion

Next, identify whether Coelenterates are triploblastics. In this case, Coelenterates are diploblastic, not triploblastic. This means they only have two germ layers (ectoderm and endoderm) during their development, not three.

Step 3: Analyze the Reason

Lastly, determine whether Coelenterates contain a mesoderm layer in between their ectoderm and endoderm layers. As previously mentioned, Coelenterates are diploblastic and only have ectoderm and endoderm layers, which means they do not have a mesoderm layer. So, this statement is incorrect.

Key Concepts

Diploblastic vs Triploblastic

To understand the assertion about Coelenterates being triploblastic, it's essential to grasp the terms 'diploblastic' and 'triploblastic'. These terms describe the layers of cells, known as germ layers, formed during the early stages of an animal's embryonic development.
Diploblastic organisms, such as Coelenterates (e.g., jellyfish and corals), possess two primary germ layers:

• **Ectoderm** - the outermost layer, which develops into the skin and nervous system.
• **Endoderm** - the innermost layer, forming the gut lining and associated structures.

In contrast, triploblastic organisms have three germ layers:

• **Ectoderm**
• **Endoderm**
• **Mesoderm**

The mesoderm is the middle layer and it contributes to forming muscles, bones, and organs. Therefore, Coelenterates, being diploblastic, lack this mesoderm layer, making the initial assertion in the exercise incorrect.

Germ Layers

Germ layers form during the early stages of embryonic development and are crucial for the differentiation of various tissues and organs in animals. This process determines the complexity of an organism's body structure.

• **Ectoderm** - gives rise to outer coverings, such as skin, and the nervous system.
• **Endoderm** - develops into internal linings, such as the gastric tract.
• **Mesoderm** - when present, forms structures such as muscles, the circulatory system, and internal organs.

In diploblastic organisms like Coelenterates, only the ectoderm and endoderm are present, limiting their structural complexity compared to triploblastic organisms, which have all three layers. This presence or absence of the mesoderm is a significant evolutionary point of divergence, leading to the development of more complex body systems in triploblastic animals.

Embryonic Development

Embryonic development is a dynamic process during which cells undergo extensive division, migration, and differentiation to form an organism. In the earliest phases of this process, the formation of germ layers takes place, setting the foundation for the animal's overall structure.

• **Diploblastic organisms** - The embryonic development involves the formation of two germ layers - ectoderm and endoderm. These layers establish a simpler organism design, as seen in coelenterates.
• **Triploblastic organisms** - In these animals, the embryonic process includes the creation of a third layer, the mesoderm, leading to greater complexity. This complexity supports advanced organ systems and body functions.

Understanding the distinctions in embryonic development between diploblastic and triploblastic organisms provides insights into the vast diversity seen in the animal kingdom and their evolutionary progress.