Final answer:
Sea squirts have a rudimentary nervous system which is a nerve net of connected nerve cells spread across their body, becoming simpler as they mature and turn sessile.
Step-by-step explanation:
Sea squirts, part of the subphylum Tunicata, have a nervous system that is simpler in comparison to more complex invertebrates and vertebrates. During their larval stage, sea squirts possess a primitive central nervous system (CNS), which includes a simple brain and neural tube. As adults, their nervous system becomes even more rudimentary, reducing to a nerve net, which is a decentralized system of connected nerve cells found across the body. This simplification occurs as sea squirts transition to a sessile (non-moving) lifestyle.
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