Cephalopoda

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(Evolutionary History: paraphyly of early forms, Nautiloidea valid.)
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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 16 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]].
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 16 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 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 [[ganglion|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 [[retina]]l 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.
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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.
As with vertebrates, the brain is partitioned into different areas that control particular functions.  For example, the brain centres for both forward swimming and closing of the suckers are located in the cerebral ganglia.  These animals are most intelligent and highly evolved of the mollusks, indeed they are the most intelligent of all the invertebrates, exhibiting complex patterns of behavior.  Octopods can easily be trained to distinguish between classes of objects.  See [http://en.wikipedia.org/wiki/Cephalopod_intelligence Cephalopod intelligence]
As with vertebrates, the brain is partitioned into different areas that control particular functions.  For example, the brain centres for both forward swimming and closing of the suckers are located in the cerebral ganglia.  These animals are most intelligent and highly evolved of the mollusks, indeed they are the most intelligent of all the invertebrates, exhibiting complex patterns of behavior.  Octopods can easily be trained to distinguish between classes of objects.  See [http://en.wikipedia.org/wiki/Cephalopod_intelligence Cephalopod intelligence]

Revision as of 12:35, 1 August 2009

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