Here's why:
* Lack of fins: Amphioxus lack the specialized fins and tail structures that most fish use for efficient swimming. They have a single, fin-like structure along their dorsal side, but it's not as powerful or maneuverable as fins.
* Muscular contractions: Amphioxus swim by undulating their body muscles, which creates a wave-like motion that propels them forward. This method is less efficient than the powerful tail movements of fish.
* Limited speed and agility: While they can swim, they are generally slow and don't have the same level of maneuverability as fish. They tend to stay buried in the sediment for much of the time.
In summary: Amphioxus are more adapted to burrowing and filter feeding than to active swimming. They can move through water, but not with the speed, agility, and grace of a typical aquarium fish.