In which of the following phylum distinct Cephalization is absent ?

Options

A. Mollusca
B. Arthropoda
C. Chordata
D. Echinodermata

Answer

To determine in which phylum distinct cephalization is absent, we need to understand what cephalization is and examine the provided options.

### Step-by-Step Solution:

1. **Understanding Cephalization**:
- Cephalization refers to the evolutionary trend where nervous tissue, over time, becomes concentrated toward one end of the organism, forming a head with sensory organs. This is common in many animal phyla.

2. **Analyzing the Options**:
- **Option A: Arthropoda**: This phylum includes insects, arachnids, and crustaceans, which exhibit distinct cephalization with a well-defined head.
- **Option B: Chordata**: Members of this phylum, including mammals, birds, reptiles, amphibians, and fish, also show clear cephalization with a pronounced head region.
- **Option C: Icaenodermata**: This phylum includes echinoderms such as starfish and sea urchins. Echinoderms do not exhibit distinct cephalization; they have a more radial symmetry and lack a centralized head structure.
- **Option D: Porifera**: This phylum includes sponges, which do not have true tissues or organs and do not exhibit cephalization.

3. **Identifying the Correct Answer**:
- Among the options, Icaenodermata (Echinodermata) is the phylum where distinct cephalization is absent. Therefore, the correct answer is **Icaenodermata**.

### Final Answer:
The phylum in which distinct cephalization is absent is **Icaenodermata**.