THE EYES OF CEPHALOPODS l6l 



by having a spiral shell in the larva which is replaced in the adult by a 

 straight, tubular shell. 



Fossil shells of gasteropods are found in the earliest Palaeozoic rocks, 

 and it is probable that the animals contained within them differed little 

 from those living at the present time, and that tentacles, ocelli, and stato- 

 cysts were developed in them in the same relative positions as in living 

 animals of the same type. 



The Scaphopoda 



These are worm-like molluscs and include the elephant-tusk shells, 

 or Dentalium. The animal is enclosed in a slightly curved tubular shell 

 which resembles in form an elephant's tusk or tooth. No eyes are found 

 in the adult animal, but statocysts are present in the larva of Dentalium, 

 which passes through typical trochophore and veliger stages. 



The Eyes of Cephalopods 



These vary from simple optic pits containing sea-water such as are 

 present in Nautilus, to the highly organized eyes of the cuttlefish, squids, 

 and octopuses. 



We shall commence our description with the simplest form, namely 

 that of Nautilus. The eyes are of large size and paired. When the 

 animal is swimming in the water, they appear in a triangular space on 

 each side between the hood above, the bases of the tentacles in front, and 

 the edges of the mantle and opening of the shell below (Fig. 113, p. 153). 

 Each eye has a central opening which leads directly into the cavity of 

 the optic vesicle, which is thus filled with sea-water. From this a shallow 

 groove runs downward to a rim-like fold which surrounds the lower 

 part of the eye. The position of this groove suggests the foetal or choroidal 

 fissure of a vertebrate eye. There has, however, been no inversion of 

 the outer wall of an optic vesicle, such as occurs in the eyes of vertebrates, 

 and the wall of the optic vesicle in Nautilus consists of a single layer, 

 whereas the secondary optic vesicle of the lateral eyes of vertebrates is 

 bilaminar. Although the development of Nautilus has not, so far as we 

 are aware, yet been described, it may be assumed that the eye is developed 

 by inversion of the superficial layer of the epidermis, so as to form a simple 

 pit as it does in the initial stage of development in other molluscs. 



The microscopic structure of the adult eye of Nautilus has been 

 described by Henle. The wall of the cup consists from within outwards 

 (Fig. 112, p. 152) of a bacillary layer formed by the inner ends of the retinal 

 cells, a pigment layer, outside which is a single layer of large nuclei con- 

 tained in the basal part of the retinal cells, this is succeeded by an 

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