Decapodiforms have two types of eyes: those with and those without a cornea.The cornea is essentially an eyelid that has become transparent and turgid and permanently covers the lens. The opening of the eyelid is reduced to a microscopic pore or is absent. Squids without a cornea have the lens exposed to seawater. The latter squids, however, have a functional eyelid.
Figure. Dorsal view of the head of the squid, Sepioteuthis australis (Fam. Loliginidae) which has a cornea. The lens, which is partially covered by a complex iris, can be seen inside the cornea. Photograph, by Mark Norman.
Figure. Dorsal view of the head of the squid Stigmatoteuthis hoylei (Fam. Histioteuthidae) which lacks a cornea. This squid that has eyes of two different sizes which makes the lens easier to see on the large eye. Photograph of by R. Young.
Squids without a cornea have an eyelid that can close over the lens. The eyelid can be transparent and is occasionally confused with a cornea. However, a large, complex slit remains that marks the edges of the closed eyelids. The pictures below show a squid with a partially closed lid on the left and a squid with a completely closed eyelid on the right.
Figure. Side views of the head of the squid Ommastrephes bartramii. Photographs of by R. Young.
