<Li> Family † Plesioteuthididae (incertae sedis) </Li> <P> Squid have differentiated from their ancestral molluscs such that the body plan has been condensed antero - posteriorly and extended dorso - ventrally . What before may have been the foot of the ancestor is modified into a complex set of tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates . </P> <P> The ancestral shell has been lost, with only an internal gladius, or pen, remaining . The pen is a feather - shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment . It is made of a chitin - like material . </P> <P> The main body mass is enclosed in the mantle, which has a swimming fin along each side . These fins, unlike in other marine organisms, are not the main source of locomotion in most species . </P>

Where is the pen located in a squid
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