THE CEPHALOPODA 319 



or oval or often a reiiifonn 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 

 Arthropod a. 



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 (e.g. Loligo media) or smaller 

 than the females, but in Nautilus the cephalic hood and the 



