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Cephalopods are considered mollusks due to homologies such as a shell, a muscular foot, and a mantle. But, they also have closed circulatory systems, two giant eyes, and eight prehensile legs. When did Cephalopoda get classified as part of Mollusca? Also, in a case like this, is there a way to quantify homologies pointing to one classification vs. another?

In researching this question I came across this blog that states Linnaeus taxonomized Octopus and Octopoda in 1758, though there is conflicting information on Wikispecies here and here

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  • $\begingroup$ The title of your question, asks “who”, its text asks “when”, but then answers that, but you actually seem to want to know “why”. It would be good if you could edit your question to clarify. $\endgroup$
    – David
    Jan 17 at 23:40
  • $\begingroup$ This seems more suitable for our sister-site, History of Science and Mathematics as it's not about the biology itself, but about an action taken in times past by a person, committee or other body of people. Flagging for migration. $\endgroup$ Jan 18 at 6:51
  • $\begingroup$ why focus on the differences and not the similarities like radula or circular brains, of course it hard to see how groups are related if you only look at the apomorphys. $\endgroup$
    – John
    Jan 19 at 2:14

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Modern classification (cladistics) is according to common descent as the backbone for our entire understanding of biology, superseding previous classification schemes based on observed traits/features.

Cephalopods are mollusks because they share a common ancestor with other mollusks; if we want Mollusca to be a clade, it must include the cephalopods. If we didn't treat cephalopods as mollusks, we'd be missing part of the "family tree" and it wouldn't be a clade. We already have a separate name for them if we want to distinguish them from other mollusks based on the properties you describe: cephalopod.

Sharing features like a shell, muscular foot, and mantle are hints that there may be common descent rather than a means for classification by themselves in modern taxonomy.

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  • $\begingroup$ cephalopods are inferred to share a common ancestor with mollusks due to evidence like homologies, or as you say, hints. i'm asking for the basis of that classification, who placed cephalopoda within mollusca, and when $\endgroup$
    – imrobert
    Jan 17 at 22:45
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    $\begingroup$ @imrobert If you literally mean who, Cuvier: hsm.stackexchange.com/q/13664/13861 that question has already been asked and answered on the history of science site. $\endgroup$
    – Bryan Krause
    Jan 17 at 23:37

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