This phylum shows the presence of nerve cells and absence of nerves

The correct option is Coelenterata.

Here is a step-by-step educational explanation of why this option is correct:

1. Understanding the Evolutionary Transition:
In the animal kingdom, the evolution of the nervous system begins with the transition from organisms that have no specialized nerve cells to those that do. In Phylum Porifera (sponges), there are no specialized nerve cells or nervous tissues at all. Instead, they rely on simple cellular coordination.

2. Nerve Cells vs. Nerves:
Phylum Coelenterata (also known as Cnidaria, which includes organisms like Hydra and jellyfish) represents the first evolutionary step where specialized nerve cells (neurons) appear. These nerve cells form a diffuse, non-polarized network called a nerve net.

However, in Coelenterates, these nerve cells are not organized or bundled into distinct structures called nerves or nerve cords. The accumulation of nerve fibers into true nerves and central processing regions (like ganglia or a brain) only appears later in evolutionary history (starting with Phylum Platyhelminthes).

3. Conclusion:
Therefore, Coelenterata is the unique phylum characterized by the presence of individual nerve cells forming a primitive nerve net, but with the complete absence of organized nerves.