The following questions refer to the description below.
Terry catches a ray-finned fish from the ocean and notices that attached to its flank is an equally long, snakelike organism. The attached organism has no external segmentation, no scales, a round mouth surrounded by a sucker, and two small eyes. Terry thinks it might be a marine leech, a hagfish, or a lamprey.
-Terry detaches the snakelike organism from the fish and uses a knife to cut off its head. In doing so, its brain slides out onto the deck of the boat. Terry peers into the cut end of the head and notices that the brain had lain in a sort of panlike structure that only partially surrounded the brain. What is the structure Terry is observing, and what is it made of?