<P> The inside surfaces of the arms and tentacles are lined with hundreds of subspherical suction cups, 2 to 5 cm (0.79 to 1.97 in) in diameter, each mounted on a stalk . The circumference of these suckers is lined with sharp, finely serrated rings of chitin . The perforation of these teeth and the suction of the cups serve to attach the squid to its prey . It is common to find circular scars from the suckers on or close to the head of sperm whales that have attacked giant squid . </P> <P> Each tentacular club is divided into three regions--the carpus ("wrist"), manus ("hand") and dactylus ("finger"). The carpus has a dense cluster of cups, in six or seven irregular, transverse rows . The manus is broader, closer to the end of the club, and has enlarged suckers in two medial rows . The dactylus is the tip . The bases of all the arms and tentacles are arranged in a circle surrounding the animal's single, parrot - like beak, as in other cephalopods . </P> <P> Giant squid have small fins at the rear of their mantles used for locomotion . Like other cephalopods, they are propelled by jet--by pulling water into the mantle cavity, and pushing it through the siphon, in gentle, rhythmic pulses . They can also move quickly by expanding the cavity to fill it with water, then contracting muscles to jet water through the siphon . Giant squid breathe using two large gills inside the mantle cavity . The circulatory system is closed, which is a distinct characteristic of cephalopods . Like other squid, they contain dark ink used to deter predators . </P> <P> The giant squid has a sophisticated nervous system and complex brain, attracting great interest from scientists . It also has the largest eyes of any living creature except perhaps the colossal squid--up to at least 27 cm (11 in) in diameter, with a 9 cm (3.5 in) pupil (only the extinct ichthyosaurs are known to have had larger eyes). Large eyes can better detect light (including bioluminescent light), which is scarce in deep water . The giant squid probably cannot see colour, but it can probably discern small differences in tone, which is important in the low - light conditions of the deep ocean . </P>

Where is the water jet located on a squid
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