Squid
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| Squid Fossil range: (at least) Late Cretaceous–Recent[1] | ||||||||||||
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Squid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two longer tentacles arranged in pairs. (The only known exception is the bigfin squid group, which have ten very long, thin arms of equal length.)
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[edit] Modification from ancestral forms
Squid have differentiated from their ancestral molluscs in such a way that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What before may have been the foot of the ancestor is now modified into a complex set of tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates.
The shell of the ancestor has been lost, with only an internal gladius, or pen, remaining. The pen is a feather-shaped internal structure which supports the squid's mantle and serves as a site for muscle attachment. It is made of a chitin-like substance.
[edit] Anatomy
The main body mass of the squid is enclosed in the mantle, which has a swimming fin along each side. These fins, unlike in other marine organisms, are not the main source of locomotion in most species.
The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings, making it effectively invisible. The underside of the squid is also almost always lighter in color than the topside, to provide camouflage from both prey and predator.
Under the body are openings to the mantle cavity, which contains the gills (ctenidia) and openings to the excretory and reproductive systems. At the front of the mantle cavity lies the siphon, which the squid uses for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the siphon in a fast, strong jet. The direction of the siphon can be changed, to suit the direction of travel.
Inside the mantle cavity, beyond the siphon, lies the visceral mass of the squid, which is covered by a thin, membranous epidermis. Under this are all the major internal organs of the squid.
[edit] Nervous system
The giant axon of the squid, which may be up to 1 mm in diameter in some larger species, innervates the mantle and controls part of the jet propulsion system.
As a cephalopod, squid exhibit relatively high intelligence among invertebrates. For example, groups of Humboldt squid hunt cooperatively, using active communication. (See Cephalopod intelligence.)
[edit] Reproductive system
In female squid, the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills. There are also red-spotted accessory nidamental glands. Both of these organs are associated with manufacture of food supplies and shells for the eggs. Females also have a large translucent ovary, situated towards the posterior of the visceral mass.
Male squid do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac. In mature males, this sac may contain spermatophores, which are placed inside the mantle of the female during mating.
[edit] Digestive system
Squid, like all cephalopods, have complex digestive systems. Food is transported into a muscular stomach, found roughly in the midpoint of the visceral mass. The bolus is then transported into the caecum for digestion. The caecum, a long, white organ, is found next to the ovary or testis. In mature squid, more priority is given to reproduction and so the stomach and caecum often shrivel up during the later stages of life. Finally, food goes to the liver (or digestive gland), found at the siphon end of the squid, for absorption. Solid waste is passed out of the rectum. Beside the rectum is the ink sac, which allows a squid to discharge a black ink into the mantle cavity at short notice.
[edit] Cardiovascular system
Squid have three hearts. Two branchial hearts, feeding the gills, each surrounding the larger systemic heart that pumps blood around the body. The hearts have a faint greenish appearance and are surrounded by the renal sacs - the main excretory system of the squid. The kidneys are faint and difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver. The systemic heart is made of three chambers, a lower ventricle and two upper auricles.
[edit] Head
The head end of the squid bears 8 arms and 2 tentacles (species in the bigfin squid group have 10 identical arms), each a form of muscular hydrostat containing many suckers along the edge. These tentacles do not grow back if severed. In the mature male squid, one basal half of the left ventral tentacle is hectocotylised — and ends in a copulatory pad rather than suckers. It is used for intercourse between mature males and females.
The mouth of the squid is equipped with a sharp horny beak mainly made of chitin[2] and cross-linked proteins, and is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species.[3] Captured whales often have squid beaks in their stomachs, the beak being the only indigestible part of the squid. The mouth contains the radula (the rough tongue common to all molluscs except bivalvia and aplacophora).
The eyes, found on either side of the head, each contain a hard lens. The lens is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does.
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