Nonchordates having mesodermal endoskeleton are

Options

A. molluscs
B. arthropods
C. annelids
D. echinoderms

Answer

To answer the question about nonchordates having mesodermal endoskeleton, let's analyze each option step by step:

### Step 1: Understand Nonchordates
Nonchordates are animals that do not possess a notochord, which is a flexible rod-like structure that is present in chordates. Nonchordates can have various types of body structures, including endoskeletons and exoskeletons.

**Hint:** Remember that nonchordates lack a notochord, which is a key characteristic that differentiates them from chordates.

### Step 2: Analyze Each Option
1. **Mollusks**: Mollusks do not have a true endoskeleton or exoskeleton. They have a soft body and may have a shell, but this is not considered an endoskeleton.

**Hint:** Consider the body structure of mollusks and whether they possess any form of skeleton.

2. **Arthropods**: Arthropods possess an exoskeleton made of chitin, but they do not have an endoskeleton.

**Hint:** Think about the external structure of arthropods and how it differs from an endoskeleton.

3. **Annelids**: Annelids (such as earthworms and leeches) do not have a true endoskeleton or exoskeleton. They have a hydrostatic skeleton, which is formed by the fluid-filled coelom.

**Hint:** Recall the body structure of annelids and their lack of a traditional skeleton.

4. **Echinoderms**: Echinoderms, such as starfish and sea urchins, do have a mesodermal endoskeleton made up of calcareous plates. This is the only option that fits the criteria of having a mesodermal endoskeleton.

**Hint:** Identify which group of nonchordates is known for having a unique internal skeleton structure.

### Step 3: Conclusion
After analyzing all the options, the correct answer is **Echinoderms**. They are the only nonchordates listed that possess a mesodermal endoskeleton.

**Final Answer:** Echinoderms