Slide showing image of 'gills of a King-crab'. Slide shows labelled diagram of 'Fig. 64. - Section through the operculum and gills of a King-crab, Limulus. x about 16.' Label below reads, 'The normal number of gills in a Limulus is five, the section from which this drawing is made showed only four.' Labelled parts include: '1. Operculum, 2. 2nd gill-book, 3. Muscle which moves gills up and down, 4. Blood-vessels, 5. Muscle which raises the operculum.' Late 19th - early 20th century