Note: 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}.