THE CEPHALOPODA 319 
31 
or oval or often a reniform pupil (Loligo, Sepia, Octopoda, Fig. 
288). A second more superficial fold forms an external false 
cornea, bounding the “anterior chamber” of the eye: the edges 
of this fold are not united in the Oigopsida but surround a wide 
orifice in the optic axis; in the rest of the Decapoda and in the 
Octopoda the edges of the fold unite and completely close in 
the anterior chamber, but in several cases a very small hole or 
“lacrymal pore” is left (Sepiola, Sepia). Finally, another fold, 
external to the false cornea, forms a transverse or inferior eyelid ; 
this structure is best developed in the Octopoda, in which group 
the eye can be completely covered in by the contraction of the 
circular orifice of the eyelid. 
The retina really consists of a single layer of cells surmounted 
by rods or rhabdomes, but the latter are extremely long, so that 
the retina is very thick. Each rhabdome is in relation to at least 
four retinal cells, whose prolongations extend into its interior, and 
each of these latter is related to two rhabdomes. A limiting layer 
of special cells is formed at the level where the retinal cells are 
joined to the rods. Below this limiting layer pigment is distributed 
through the retinal cells, especially in their lower ends and towards 
their upper extremities: in the dark all the pigment granules are 
collected at the bases of the cells, as in the Vertebrates and 
Arthropoda. 
The cuticular crystalline lens is the product of both the internal 
and external surfaces of the true cornea. Its two segments are 
formed of successive concentric layers. The external segment is 
the less prominent of the two; the internal segment, which cor- 
responds morphologically to the crystalline lens of Gastropoda, is 
much more convex and larger, but it does not occupy the whole 
of the ocular cavity or “posterior chamber” of the eye. The 
remainder of the cavity is filled by a semi-fluid vitreous body, 
as is the case in the majority of the Gastropoda. The eye of 
Dibranchia can be accommodated for near and distant vision by 
variation of the distance between the lens and the retina; con- 
sequently these animals are never presbyopic. 
In a few genera of Cephalopods there are sensory organs which 
appear to be thermoscopic eyes. They are situated beneath the 
integument, and in Cheiroteuthis grimaldii are found on the ventral 
side of the body and on the dorsal aspect of the fins. These 
organs consist of a large lenticular and highly pigmented chromato- 
phore, beneath which is a flattened nerve-ending, surrounded by 
large transparent cells. 
5. Reproductive Apparatus.—In all the Cephalopoda the sexes are 
separate, and sometimes there is a well-marked sexual dimorphism. 
As a rule the males are more slender (¢.g. Loligo media) or smaller 
than the females, but in Nuutilus the cephalic hood and the 
