360 SEA-SIDE STUDIES. 



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 cover- 

 ing the eyes ? A second requisite is a special ganglion, 

 or centre of sensation ; and even this is wanting in 

 many cases. In Pleurohranchus and Aplysia I find 

 the optic nerve arising from the ganglion which sup- 

 plies 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 

 reception of luminous influence, enabling the animal to 

 distinguish light from darkness, not only in the gen- 

 eral way, like a blind man conscious of a change of tem- 

 perature in passing from sunlight into shade, but also 

 in the special way of minute local variations, such as 

 are caused by the shadows of near objects. 



I remember once being seated with a philosophic 

 friend, and much bored by the presence of a morning 

 caller — a large white-waistcoated man, " such an ass, 

 and so respectable ! '' stiff with ignorance and haughti- 

 ness : the kind of man who seems afraid of lowerlnof 

 his eyebrow lest it should crease his cravat. He droned 

 away about " the house " and Lady Jane, about his 



* Leydig: Histologie d. Mensch. u. Thiere, lSo7, p. 249. I have also 

 observed this in a species of Doris of which the name is unknown to 

 me. In general the Doridoe have minute optic ganglia. 



