Phanerophthalmus cylindricus has a translucent shell. Juveniles have scattered brown spots on the body with orange spots showing through the shell. Adults becomes brown to olive-green spots and brown spots showing through the shell.
Phanerophthalmus cylindricus may be distinguished from the other members of the genus by its larger shell.
This common Headshield slug is nocturnal and can be found in protected rocky habitats at depths of 1-9 m.Sometimes it can be found in Halimeda kanaloana beds.
The eggs mass is lozenge-shaped and densely packed with eggs.