342 SEA-SIDE STUDIES. 



the skin and muscles, and rest on the brain (oesophageal 

 ganglia), attached thereto by a microscopic nerve. There is 

 no aperture in the skin, as there is in ours, through which 

 the rays of light may fall directly on the eye ; so that in 

 spite of pigment, lens, and nerve — the essential parts of a 

 visual organ — vision is utterly impossible ; as you may con- 

 vince yourself even with your own admirable eyes, if the lids 

 are obstinately closed over them. I am aware that claii- 

 voyants of the strictly unveracious species, profess to see 

 with their eyes closed ; but our simpler Molluscs have no 

 such pretensions ; they have not yet given in to the clair- 

 voyant mania, and are content to submit to those laws of 

 physics which regulate phenomena with the same unerring 

 consistency in the world of Naked-gills as in that of Clothed 

 Noodles. 



A first requisite in vision is surely the formation of an 

 image ; and how can this image be formed when the rays have 

 to pass through the skin and muscles covering the eyes ? A 

 second requisite is a special ganglion, or centre of sensation ; 

 and even this is wanting in many cases. In Pleurobranchus 

 and Aplysia I find the optic nerve arising from the gan- 

 glion which supplies the antennae ; and Leydig says the Doris 

 luguhris has its small eyes resting immediately on the 

 brain.* Nevertheless, although these eyes are incompetent 

 to vision, they represent the early stages of that marvellous 

 and complex function ; they are special organs for the recep- 



• Letdio : Ilistoloijie d. Mensch. u. Thieve, 1857, p. 249. I have also ob- 

 served tliis in a species of Doris of which the name is unknown to me. In 

 gencml the Doridce have minute optic ganglia. 



