Figure 4. Squid nervous system anatomy. (a). Squid are principally pelagic decapodiforms. Their rigid mantle and swimming fins aid with fast locomotion. (b) and (b’). Stellate ganglion anatomy of Doryteuthis pealeii, courtesy of Paloma Gonzalez-Bellido and Trevor Wardill. See also Figure 5. Second-order fibers of
the squid escape system arrive at the stellate ganglion (SG) via the stellate connective (SC), a branch of the pallial nerve (PN). These second-order fibers form synapses with processes of third-order cells, which are located in the giant fiber lobe (GFL). Third-order fibers exit the stellate ganglion through one of the stellar nerves (asterisk). The giant axon, found in the last stellar nerve (SN), is the third-order cell with the greatest diameter. This dissection also shows the fin nerve (FN) and fin connective (FC), which control skin
coloration and are not part of the escape system per se. (c) and (c’). Dorsal view of the central brain and
optic lobes of Euprymna scolopes, specimen courtesy of Mark Mandel. The squid has vertical lobes (VL) that join at the midline and large optic lobes (OL). The statocysts (white arrowheads) remain attached to the cerebral cartilage. (d). Coronal view of the optic lobe of Euprymna scolopes, specimen courtesy of Bethany Rader. The outer layers of the squid optic lobe (1–3) follow octopus organization, but squid have an additional inner plexiform layer (4; Young, 1974). 1: outer granule cell layer; 2: outer plexiform layer; 3: inner granule cell layer; 4: inner plexiform layer. H&E-stained.