With regard to general structure, cephalopods have three easily distinguished regions. From front to rear, these regions are: (1) the brachial crown (arms and tentacles) surrounding the mouth; (2) the head, with prominent lateral eyes; and (3) the mantle, which may have a pair of fins on the sides. This overall three-part structure is less distinct in nautilids but is still recognizable. Nautilids have approximately 60 arms (sometimes called tentacles or cirri) arranged in two rings around the mouth. Neocoleoids have a single ring of either eight or ten appendages surrounding the mouth. On those that have ten, two appendages are modified into either ven-trolateral tentacles (decapods) or dorsolateral velar filaments (vampires). Thus, all living cephalopods other than the nau-tilids have eight arms, and some have either two additional tentacles or two filaments. Although some nonspecialists refer to all cephalopod appendages as tentacles, one should
avoid this usage because it confuses the differentiation of the specialized appendages in decapods and vampire squids.
Cephalopods range in size from the giant squids, Archi-teuthis spp., which are commonly longer than 6.56 ft (2 m) in mantle length (ML) and reportedly reach 16.4 ft (5 m) in ML, 59 ft (18 m) in total length, and 661.3 pounds (300 kg) in weight, to tiny species like Idiosepius spp., decapods which mature at an ML of about 0.23-0.31 in (6-8 mm); or the sexually dimorphic pelagic octopods, Argonauta spp., in which mature males are only 0.39 in (1 cm) in ML. Several squid species and at least two species of octopods reach sizes larger than an adult human. They include the true giant squid as well as the muscular (and dangerous) ommastrephids, the weakly muscled cranchiids and chiroteuthids, and the giant Pacific octopus or "devilfish."
Many species of squids, cuttlefishes, and octopods can radically change their appearance within a fraction of a second. This ability to transform themselves so rapidly is the reason that cephalopods have been called masters of disguise. These remarkable transformations result from interactions among brown, red and yellow pigment-filled chromatophore (color-producing) organs under the control of the animal's nervous system; reflective iridophores (cells that produce a silvery or iridescent pigment); fixed white leucophores (pigment cells containing guanine); fixed tubercles (small nodules); and erectile muscular flaps and papillae in the animal's skin. Furthermore, some cephalopods have light-producing organs called photophores. Cephalopod photophores come in two fundamentally different types. In the first type, known as intrinsic photophores, the light is produced biochemically by the squid or octopod. The second type, called bacterial photophores, makes use of symbiotic photogenic bacteria that grow in special chambers associated with the ink sac in the host. The light produced by both types of photophores is usually blue-green in color; the color can be altered, however, by structures associated with the photophore.
Other features include an ink gland and ink sac associated with the intestine that is characteristic of neocoleoids. Complex, highly developed nervous and sensory systems are also typical of living cephalopods, although less so for the nau-tilids than for the others. Especially noteworthy are the image-forming eyes, with lenses in the neocoleoids; and the complex brain developed from the nerve ring surrounding the esophagus. Other noteworthy peculiarities, including a muscular hydrostatic skeleton and a pattern of muscle fiber alignment known as oblique striation, also are characteristics of cephalopods.
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