508 THE SENSES AND SENSORY ORGANS. 
of retinal end organs from the superficial epiblast. Lankester, 
in his monograph on the Mollusca [Encyclop. Britt., 1891], says, 
‘ The eye of Nautilus is amongst the most interesting structures 
of that remarkable animal,’ and adds: ‘It is simply a slightly 
projecting hemispherical box like a kettledrum, half an inch in 
diameter, its surface looking like that of the surrounding integu- 
ment, whilst in the middle of the drum-membrane is a minute 
hole. Owen naturally thought that some membrane had 
covered this hole in life, and had been ruptured in the 
specimens studied by him. It, however, appears from the 
studies of Hensen that the hole is a normal aperture lead- 
ing into the globe of the eye, which is accordingly filled with 
sea-water during life. There is no dioptric apparatus in 
Nautilus, and in place of refracting lens and cornea we have 
actually here an arrangement for forming an image on the 
principle of a “pinhole camera.” The cavity is solely lined by a 
naked retina, which is bathed by sea-water on one surface, and 
receives the fibres of the optic nerve on the other’ (the italics are 
mine). To my mind the above is an utterly improbable 
deduction. There is no evidence that the retina is naked and 
bathed with sea-water, later than Hensen’s paper [209]. It 
may be that a very thin epidermal layer covers it, formed like 
the primary optic depression in the vertebrate embryo by an 
involution of the epidermal epiblast, and there is no evidence 
that the retina is not developed in these animals from an out- 
growth from the central nervous system, as it is in Vertebrates. 
Analogy with Olfactory and Auditory End Organs. — It has 
been concluded that the nerve-terminals of the olfactory and 
auditory nerves are derived from the surface epiblast, and 
it has been argued that this is the usual manner in which 
nerve-end organs originate. The researches of His on the 
origin of the olfactory cells appear to indicate that they 
originate independently of the neuroblast from which the 
olfactory nerve is developed. If this is so it would certainly 
be ana priori argument in favour of a similar origin for other 
sensory nerve-terminals. It is difficult, however, to under- 
stand the want of continuity in the nervous tract which the 
