Cephalopoda

* They have, around the front of the head, a group of elongated muscular arms, which are usually furnished with prehensile suckers or hooks. The head is highly developed, with large, well organized eyes and ears, and usually with a cartilaginous brain case. The higher forms, as the cuttlefishes, squids, and octopi, swim rapidly by ejecting a jet of water from the tubular siphon beneath the head. They have a pair of powerful horny jaws shaped like a parrot's beak, and a bag of inklike fluid which they can eject from the siphon, thus clouding the water in order to escape from their enemies. They are divided into two orders, the Dibranchiata, having two gills and eight or ten sucker-bearing arms, and the Tetrabranchiata, with four gills and numerous arms without suckers. The latter are all extinct except the Nautilus. See Octopus, Squid, Nautilus.