5o8 THE SENSI-.s 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 tin- 

 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 bv a 

 naked retina, scinch 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 tin- 

 olfactory nerve is developed. If this is so it would certainly 

 In- an a priori argument in favour of a similar origin for other 

 M-nsory nerve-terminals. It is difficult, however, to under- 

 stand the want of continuity in the nervous tract which tin- 



