Cephalopoda

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(added superordinal classification of Wade, 1988)
(Biology: Pachydiscus seppenradensis is Parapuzosia seppenradensis)
 
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==Biology==
==Biology==
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The name Cephalopoda literally means "head feet" which refers to the cluster of [[arm]]s and/or [[tentacle]]s that project forward from the head, surrounding the mouth.  The group includes living [[Coleoidea|coleoids]] (squid, octopods, and cuttlefish) and ''[[Nautilus]]'', and a large number of ancient (mostly [[Paleozoic]] and [[Mesozoic]]) forms.  All are active marine predators (although some early types were drifters), able to swim swiftly, and easily competing with fish in the marine habitat.  There are 650 living species, but more than 7,500 fossil forms are known (and as in all cases like this this number is obviously a gross underestimate of the real number of cephalopod species that have ever lived through the [[Phanerozoic]] time).  Like fish they are equipped with highly developed eyes and other sense organs, include both active swimmers and bottom-dwellers, and in many cases have a streamlined body for more efficient locomotion.  Swimming is by rapidly expelling water from the [[mantle cavity]].  The water is forced out through a funnel or [[siphon]] - the [[hyponome]] - actually a tube-like flap of modified foot, thus driving the animal in the opposite direction.  This is the key to the so-called "jet-propulsion" of these animals   The funnel is highly maneuverable and can be directed in any direction, allowing motion backwards or forwards.  However, the fastest movement is backward escape swimming, with powerful contractions of the mantle ejecting water through the forward facing funnel.  A cloud of "ink" can also be ejected as a sort of underwater smoke screen to hide the fleeing animal.
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The name Cephalopoda literally means "head feet" which refers to the cluster of [[arm]]s and/or [[tentacle]]s that project forward from the head, surrounding the mouth.  The group includes living [[Coleoidea|coleoids]] (squid, octopods, and cuttlefish) and ''[[Nautilus]]'', and a large number of ancient (mostly [[Paleozoic]] and [[Mesozoic]]) forms.  All are active marine predators (although some early types were drifters), able to swim swiftly, and easily competing with fish in the marine habitat.  There are 650 living species, but more than 7,500 fossil forms are known (and as in all cases like this this number is obviously a gross underestimate of the real number of cephalopod species that have ever lived through the [[Phanerozoic]] time).  Like fish they are equipped with highly developed eyes and other sense organs, include both active swimmers and bottom-dwellers, and in many cases have a streamlined body for more efficient locomotion.  Swimming is by rapidly expelling water from the [[mantle cavity]].  The water is forced out through a funnel or [[siphon]], knows as the hyponome, thus driving the animal in the opposite direction.  This is the key to the so-called "jet-propulsion" of these animals. The funnel is highly maneuverable and can be directed in almost any direction, allowing motion backwards or forwards.  However, the fastest movement is backward escape swimming, with powerful contractions of the mantle ejecting water through the forward facing funnel.  A cloud of "ink" can also be ejected as a sort of underwater smoke screen to hide the fleeing animal.
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All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea]], and worms.  The head projects into a crown of prehensile tentacles - ranging from 8 in the octopus to 80 or 90 in the living nautilus. These tentacles are actually a specialized form of the standard molluscan foot, and used for grasping prey. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].
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All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea|crustaceans]], and worms.  The head projects into a crown of prehensile arms ranging from 8 in octopus to about 90 in the living nautilus. Cephalopod arms, or tentacles, and used for grasping prey, are a specialized development of the basic molluscan foot. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].
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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician [[Endocerida|endocerid]] [[Nautiloidea|nautiloid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Pachydiscus seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons.  Such giant cephalopods play or played a similar ecological role of top predator to that of [[Devonian]] [[Arthrodira|arthrodire]] [[Placodermi|placoderms]], [[Mesozoic]] [[pliosaur]]s and [[Cenozoic]] [[Odontoceti|toothed whales]].
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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician orthoconic [[Endocerida|endocerid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Parapuzosia seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons.  Such giant cephalopods play or played a similar ecological role of top predator to that of Devonian [[Arthrodira]], Mesozoic [[pliosaur]]s, and Cenozoic [[Odontoceti|toothed whales]].
Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals).  There is a high level of cephalization (development and concentration of sensory and neural centers in the head).  The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes.  They are visual creatures, changing colour to express mood.  The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates.  The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.
Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals).  There is a high level of cephalization (development and concentration of sensory and neural centers in the head).  The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes.  They are visual creatures, changing colour to express mood.  The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates.  The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.

Latest revision as of 11:56, 12 September 2014

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