The most striking passages are about the sea-life round Lesbos. As well as live observation, he got specimens from the catches of fishermen. His observations on catfish, electric fish ("Torpedo") and angler-fish are detailed. His writing on cephalopods such as "Octopus", "Sepia" (cuttlefish) and the paper nautilus ("Argonauta argo") are accurate. His description of the hectocotyl arm, used in sexual reproduction, was widely disbelieved until its rediscovery in the 19th century. He separated the aquatic mammals from fish, and knew that sharks and rays were part of the group he called Selachē (Selachimorpha).