Step 10: Use the initial circle and arcs as guides to draw the rest of the mantle. Simply darken the outer edges of the initial guides to create the shape of the octopus's mantle. Add a small curved line below the eye as the opening for the siphon.
Step 11: Below the eye, draw the siphon or funnel using a few short, curved lines. Octopuses expel water out of the siphon for locomotion. The siphon should be similar to a cylinder.
Step 12: Use the long lines as guides to draw the tentacles. Start with the lower lines because they will overlap the top lines. Follow the path of the guide as you draw the shape of the tentacle around it. Use light strokes at first, and then darken the lines when you get the shape of the tentacle right. Curve the shape of the tentacle along the path of the guide. Add some curved lines on the outer edge for the suction cups. The suction cups are on the underside of the octopus's tentacles, so they should only be visible on the tip that's curved. The base of the tentacle should be thick, and it should gradually get thinner toward the tip.
Step 13: Use the line above as a guide to draw the next tentacle. Follow the path of the guide as you draw the shape of the octopus's tentacle around it. Make the base thick and the tip thin. If you chose to not curve your guide lines in the initial steps, drawing the tentacles will be a lot easier.
Step 14: Use the bottom line as a guide to draw the octopus's third tentacle. Remember to sketch the shape of the tentacle lightly at first and only darken the lines when you like the structure. Make the lines wavier for a more organic, less rigid structure.