The cephalopods evolved during the Cambrian and became dominant during the Ordovician. There are about 17,000 named species of fossil cephalopods and 800 species of living cephalopods. Some of the early forms has straight shells called orthocones that flourished in the oceans between the Ordovician (488 mya) and Triassic periods (270mya) with some reaching a length of 10 meters in length. During the Ordovician Period, straight-shelled forms developed a planispiral shape where the shell coiled in a single plane. Three main forms are identified within Cephalopoda, Ammonoidea whixh is extinct shelled form, Nautiloidea, the only living shelled genus, and Coleoides, that includes squids, cuttlefish, octopuses, and an extinct form, belemnites. Cephalopods are found in all the oceans of Earth. None of them can tolerate fresh water with the exception of the brief squid (Lolliguncula brevis) that can tolerate brackish water. Cephalopods are considered the moost intelligent of the invertebrates with a brain-to-body ratio falling between endothermic and ectothermic vertebrates. Cephalopods have advanced vision, can detect gravity and sound and have a variety of chemical sense organs. With the exception of the Nautilidae and a species of octopus, all known cephalopods have an ink sac which can expel a cloud of dark ink to confuse predators. Most cephalopods can move by jet propulsion. While jet propulsion is never the main mode of locomotion, it is helpful promoting stop-start motion and continues to provide bursts of high speed.