THE EYES OF CEPHALOPODS 163 



outermost layer of small scattered nuclei and nerve-fibres ; the latter pierce 

 an external limiting membrane and pass inwards through the optic stalk 

 to the cerebral ganglion (anterior part of the oesophageal ring). In the 

 interior of the optic cup there is neither vitreous, lens, nor iris, and as an 

 optical instrument it is comparable to a camera of the simple pill-box type. 

 Nautilus (Fig. 119) with its simple type of eyes is especially interesting 

 from the phylogenetic standpoint ; it is the only living representative of 

 the extinct Nautiloid tetrabranchiata, which were abundant in the Palaeozoic 

 epoch ; the earliest representatives being found in the Cambrian rocks. 

 It also closely resembles the Ammonites, but there are certain differences 

 in the shell, and it is doubtful whether the Ammonites were tetrabranchiate, 

 like Nautilus, or dibranchiate. This difference, however, is not so 



Fig. 120. — Cartilaginous Internal Skeleton of Nautilus pompilius. 



fundamental as might be expected from the differences in structure which 

 exist between the branchiae and eyes of living representatives of these 

 fossils, since there are many points of agreement between dibranchiate and 

 tetrabranchiate cephalopods which outweigh such differences as have 

 occurred in the long geological period which has elapsed since the fossil 

 representatives of the living genera were alive. Incidentally one may men- 

 tion, as an example of an important structural similarity between these two 

 widely separated orders, the occurrence of a cartilaginous endoskeleton — the 

 " cranial cartilage," Fig. 120 — which is present in both Nautilus and 

 Sepia. It is noteworthy that the cranial cartilages of Nautilus and Sepia 

 resemble in certain respects the endoskeleton of some chaetopods, Crustacea, 

 and arachnids, e.g. the entosternite of Scorpio and Limulus. 



