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Reference.org
Cirrate shell
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<p><a href="/facts/Cirrina/suStthkk">Cirrate octopuses</a> possess a well-developed internal shell that supports their muscular <a href="/facts/Cephalopod_fin/Wbow7sNC">swimming fins</a>. This is in contrast to the more familiar, finless, <a href="/facts/Incirrina/SXDwxuV5">incirrate octopuses</a>, in which the shell remnant is either present as a pair of <a href="/facts/Stylet_(anatomy)/gp8rFeAW">stylets</a> or absent altogether. </p><p>The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms. It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to <a href="/facts/Cartilage/vxy2qzKN">cartilage</a>. Each of the eight extant cirrate <a href="/facts/Genus/m8EFjSms">genera</a> is characterised by a distinct shell morphology outlined below (below taxonomy updated per <a href="/facts/World_Register_of_Marine_Species/lfmyq6bH">WoRMS</a>): </p> <ul><li>Superfamily Cirroteuthoidea <ul><li><a href="/facts/Cirroteuthidae/lEb1C1Kp">Cirroteuthidae</a> <ul><li><i><a href="/facts/Cirroteuthis/DUSMXWwt">Cirroteuthis</a></i> — saddle-shaped, with large wings</li> <li><i><a href="/facts/Cirrothauma/JxRchNAK">Cirrothauma</a></i> — butterfly-shaped</li></ul></li> <li><a href="/facts/Stauroteuthidae/ueWV0evB">Stauroteuthidae</a> <ul><li><i><a href="/facts/Stauroteuthis/ueWV0evB">Stauroteuthis</a></i> — U-shaped</li></ul></li></ul></li> <li>Superfamily Opisthoteuthoidea <ul><li><a href="/facts/Opisthoteuthidae/C5yYPrM5">Opisthoteuthidae</a> <ul><li><i><a href="/facts/Opisthoteuthis/NLTY0SXM">Opisthoteuthis</a></i> (also <i>Exsuperoteuthis</i> & <i>Insigniteuthis</i>)— U-shaped, lateral wings usually tapering to fine points but termination complex in certain species</li></ul></li> <li><a href="/facts/Cirroctopodidae/c7aHkg6X">Cirroctopodidae</a> <ul><li><i><a href="/facts/Cirroctopus/c7aHkg6X">Cirroctopus</a></i> — V-shaped, lateral wings tapering to fine points</li></ul></li> <li><a href="/facts/Grimpoteuthidae/GXFtcyfR">Grimpoteuthidae</a> <ul><li><i><a href="/facts/Grimpoteuthis/JnqETdqD">Grimpoteuthis</a></i> — U-shaped, lateral wings ends expanded in a broad lobe (with offset spike present or absent).</li> <li><i><a href="/facts/Luteuthis/znhhTrSz">Luteuthis</a></i> — W-shaped, lateral wings ends expanded (with offset spike present).</li> <li><i><a href="/facts/Cryptoteuthis/I9Krz8MU">Cryptoteuthis</a></i> — U-shaped, each lateral wing ending in broad lobe with pointed projection.</li></ul></li></ul></li></ul> <p>The comparatively simple shells of <a href="/facts/Opisthoteuthidae/C5yYPrM5">Opisthoteuthidae</a> and <a href="/facts/Stauroteuthidae/ueWV0evB">Stauroteuthidae</a> are thought to approximate the <a href="/facts/Plesiomorphy/n5K8vSKd">ancestral</a> shape, with those of <a href="/facts/Cirroteuthidae/lEb1C1Kp">Cirroteuthidae</a> being more <a href="/facts/Derived_trait/t21GWRl1">derived</a>. The shell of <i><a href="/facts/Cirroctopus/c7aHkg6X">Cirroctopus</a></i> appears transitional in form between those of incirrate octopuses and other cirrates, and resembles the reduced shell of the <a href="/facts/Late_Cretaceous/pcHa4fBg">Late Cretaceous</a> <i><a href="/facts/Palaeoctopus_newboldi/jlVR8jTz">Palaeoctopus newboldi</a></i>. The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell. </p>