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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.Cephalopods are the only mollusks with a closed circulatory system.Cephalopods occupy most of the depth of the ocean, from the abyssal plain to the sea surface.Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N). The giant nerve fibers of the cephalopod mantle have been widely used for many years as experimental material in neurophysiology; their large diameter (due to lack of myelination) makes them relatively easy to study compared with other animals.Most cephalopods possess an assemblage of skin components that interact with light.These may include iridophores, leucophores, chromatophores and (in some species) photophores.The study of cephalopods is a branch of malacology known as teuthology.Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids.
Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites). None of them can tolerate freshwater, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, is a notable partial exception in that it tolerates brackish water.
Coleoids have two gill hearts (also known as branchial hearts) that move blood through the capillaries of the gills.
A single systemic heart then pumps the oxygenated blood through the rest of the body.
The ejected cloud of melanin is usually mixed, upon expulsion, with mucus, produced elsewhere in the mantle, and therefore forms a thick cloud, resulting in visual (and possibly chemosensory) impairment of the predator, like a smokescreen.
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).