Sea squirts are tiny (from about one millimeter to about five centimeters in diameter) sea animals that permanently affix to rocks or other underwater surfaces early in their lives. Their name comes from the tendency to reflexively squirt out a jet of water from their feeding siphons when touched. Another name for them is tunicates, from the hard "tunic" the secrete around themselves as a defensive mechanism, like a softer version of the mollusk's shells. Sea squirts are in the phylum chordata; just like human beings, during some part of their life they have a spinal cord (though no vertebrae!) and support structure for it.

Since they are chordata, they are reasonably close to humans on the evolutionary scale -- closer than jellyfish or earthworms, for example. This proximity in design makes them useful for scientific study; researchers in the UK have used a sea squirt model to research fertilization properties of human sperm and eggs. Their larvae have gill slits and tails, and at early stages look basically the same as human, rat, fish, and every other chordate embryo.

Each sea squirt has two siphons, one for taking in food, the other for expelling waste, known as the branchial and atrial siphons, respectively. They feed by forcing water through a branchial basket, something like a sieve that captures bacteria in the water. This is done constantly, and even small sea squirts can filter hundreds of gallons of water a day, removing around 95% of the bacteria from it.

Sea squirts are also known for their brain, or rather, their lack thereof. When a squirt starts out, its nervous system has two ganglia, one for controlling movement and one controlling digestion. The ganglion used for movement is called the cerebral ganglion, and is connected to the squirt's spinal cord and light sensing organ. It directs the animal to an appropriate rock to settle down on. When a rock is found, usually within the first 24 hours of the squirt's life, the young squirt attaches to it. Since the tail, spinal cord, and cerebral ganglion are no longer needed, the sea squirt reabsorbs them for energy a few minutes after attachment. The digestive nervous system (also known as the visceral ganglion) is left alone, and controls the characteristic squirting reflex.

In a piece he read on NPR in 1996, Andrei Codrescu had this to say about the sea squirt: "My friend Pat Nolan writes from California about the sea squirt, an aquatic mammal (sic) with a very simple nervous system that swims around until it finds a suitable rock or coral reef to settle in for life." And then, according to Codrescu, the sea squirt "devours its own brain ... kind of like tenure." This notion that the sea squirt "eats" its "brain" is obviously wrong: no ingestion is done, squirts have ganglia instead of brains, and a nervous system still exists after the process is complete. Codrescu's comparison to tenure, however, is fair.