The chambered nautilus (Nautilus pompilius) is the principal species of the Nautilus group of the cephalopod family. Other family members include the squid, octopus, and cuttlefish. The nautilus is the only family member with an external shell. The shell utilizes countershading to hide from predators and prey. The mottled chocolate-brown upper surface blends in with seabed and coral below and the white lower surface blends in with brighter waters above. When the nautilus hatches from its egg, it is about one inch in diameter and the spiral shell generally consists of four chambers or camerae. As the nautilus matures, new chambers are added, each larger than before, and the body takes up residence in the new chamber. The older, smaller chambers are largely sealed off, except for a small opening in the center of the dividing wall. The older chambers usually contain air to maintain buoyancy, but can be filled with seawater if the animal wants to descend. A mature animal measures about 10 inches in diameter. The nautilus has about 90 tentacles. Unlike other cephalopods, the tentacles lack suckers. Rather, they have a combination of ridges and grooves. These allow the nautilus to strongly grasp its prey and bring it to the parrot-like beak for crushing and ingestion. Unlike other cephalopods, the nautilus has very poor eyesight, the eye having no lens (similar to a primitive pin-hole camera). Its primary sensory organs seen to be smell and touch. With a 20-year lifespan, it is the longest-living of the cephalopods. It is found only in the Indian Ocean and in the western Pacific Ocean between about 30° north and 30° south. Preferring somewhat cooler waters, it resides primarily near deep slopes of coral reefs at depths between 500 and 2,000 feet. The shell implodes if the creature descends below about 2,400 feet. The shell presents one of the finest natural examples of a logarithmic spiral, with the average ratio being 1.33 to 1. The shells were popular collector items through Renaissance, with goldsmiths mounting them on stems to make decorative shell cups.