Mollusks belong to the Cephalopoda class. Among the cephalopods we find animals like the squid, the cuttlefish, the nautilus, and the octopus. One species of squid, the Architeuthis can grow to gigantic sizes. Some specimens are known to reach up to seventeen feet in length. The cuttlefish is probably the most beautiful of all chephalopods. Some cuttlefish present interesting patterns and beautiful colors. The chambered nautilus is usually called « the living fossil » since he has not suffered a lot of evolutionary changes since prehistoric times. Although the nautilus lives in deep oceans, it is possible to observe some live specimens at the Institut d'Oceanographie de Paris. The word cehalopoda is derived from the Greek words « head » and « foot ». The first animal that comes to mind after such a description is the octopus. In despite of its simple appearance, the anatomy of the octopus is very sophisticated. The octopus has a complex nervous system that is essential to his daily survival. He possesses the most developed brain from all invertebrates; he is capable of communicating, using tools, learning through cognition and observation, and even playing. .
At least two hundred different species of octopuses are known to exist. The biggest octopus ever captured "weighed more than six hundred pounds and measured thirty one feet from arm tip to arm tip." (Stewart, "Armed But Not Dangerous") .
The nervous system of the octopus consist of a complicated net of nerves that run from his brain, called a ganglion, through his entire body. The brain of the octopus contains three hundred million neurons compared to one hundred billion in humans. The octopus brain needs to constantly process large amounts of visual and tactile information, which may be the reason for its complexity.
The eyes of an octopus are similar to human eyes when we compare the crystalline and the iris.