It reacted to its meager senses without much processing

A note for The Half-Lesson, "Your Brain Is Not for Thinking," in Seven and a Half Lessons About the Brain by Lisa Feldman Barrett.
Some context from page 2‌ is:

The amphioxus didn’t need a brain. Its cells for sensing were connected to its cells for moving, so it reacted to its meager senses without much processing.

What is this?

See these references.

Much of what we know about the neural circuitry of amphioxus comes from the study of its immature (i.e., larval) stage. The immature amphioxus controls its movement with more than just a simple set of reflex pathways. Consider, for example the neural control involved when an amphioxus moves away from an object that touches its skin (called an "escape" circuit). Amphioxus skin has a meshwork of nerves and information about touch enters the nerve cord from the skin to synapse on the dendrites on neurons that belong to the mass of neurons that control its motor movements. This dense mass of interwoven neurons is called "neuropile." This neuropile also receives input from the amphioxus frontal eye spot and other neurons that help to regulate its wake-sleep cycle by detecting changes in light, and from the neurons that control balance. The activity in these neurons is most likely modulated by various sources, including dopamine (which evolved as a metabolic regulator).

These observations suggest two hypotheses:

• Amphioxi have a single clump of neurons that integrate multiple sensory inputs, which in turn control motor movements.
• These neurons may be described as having a motivational role in controlling how responsive the animal is to varying sensory inputs.