Echinodermata
The Echinodermata are a group of aquatic organisms that are known for their 5-point build types and their existance in only salt-water. There are extremely important to eco-system, as they account for most of the limestone production with their ossified skeletons. They also are known for their ability to regenerate new body parts, even in extreme conditions. Containing about 70,000 species, the Echinoderms are fairly large group of marine life.
The problem with most Echinodermata is their undeveloped nervous system. Aside from their motor skills and some eye spots, they lack most common features of a nervous system. They can feel, and they lack proper eyes and ears. Starfish have no central brain, and instead use a radial nerve that controls the whole body and is responsible for finding food and mates. Due to their de-centralized nervous system, they are able to sustain major damage and still live.
Right in line with the starfish, the sea urchin has no brain, rather, they have a radial nerve controlling the whole body. However, unlike the starfish, they have no eyespots and instead are photosensitive themselves. In simpler terms, their entire body is an eye. They observe their surrounds via their spikes, which send information throughout the body to decide if it should react or not.
The sea cucumber is also like every other Echinoderm, as it lacks a central functioning brain and instead uses one single nerve fiber that works by sending signals all through the slug. The sea cucumber is the only one of the three organisms to have no sensory organs.