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A single systemic heart then pumps the oxygenated blood through the rest of the body.
Like most molluscs, cephalopods use hemocyanin, a copper-containing protein, rather than hemoglobin, to transport oxygen.
Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids.
The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus.
However, a more sophisticated behaviour has been observed, in which the cephalopod releases a cloud, with a greater mucus content, that approximately resembles the cephalopod that released it (this decoy is referred to as a Pseudomorph).
This strategy often results in the predator attacking the pseudomorph, rather than its rapidly departing prey. The inking behaviour of cephalopods has led to a common name of "inkfish", primarily used in fisheries science and the fishing industry, paralleling the terms white fish, oily fish, and shellfish.
In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains.
Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation.
They have a simple "pinhole" eye through which water can pass.