Which one of the following animals has a nervous system but no brains?
A. Earthworm
B. Hydra
C. Amoeba
D. Cockroach

Hint: Brain is the control center of an animal and it performs different kinds of functions. The animal that has a nervous system but no brains belong to the phylum Coelenterata class Hydrozoa. It contains the nerve cells and the unpolarised nerve cells of the epidermis in the organism form the nerve net but there is no brain presence in the brain.

Step by step answer:Hydra has anatomically one of the simplest nervous systems which is a nerve net formed by the nerve cells present in the epidermis of the Hydra with no sign of a brain.
The nerve cells morphologically are of two types: sensory and ganglion neurons.
The contractions and the extension of the tentacles and the body, the different types of locomotion in Hydra like gliding, floating, walking, looping, and somersaulting are all under the control of the nervous system of Hydra.
Earthworm- it belongs to the phylum Annelida. An earthworm has a well-developed nervous system with a brain but the head is absent.
Cockroach- it belongs to the phylum Arthropoda. It has a well-developed nervous system with a nerve ring, nerve cords, and neuro-endocrine sympathetic system.
Amoeba- there is no nervous system as there is no presence of neurons. But there is a presence of different types of sensors.
Hence option B is correct.

Note: The epidermis of Hydra forms nerve cells and these unpolarised nerve cells of the epidermis from the nerve net or neurons in Hydra. These neurons are responsible for the different kinds of activities occurring in Hydra like locomotion, food capturing, offense and defense, anchorage, etc.