280 
OLFACTORY AND VISUAL ORGANS. 
the animals themselves the i)owei’ to iierceive them. Our British 
/Ii/aHiiice, especially when irritated or itlnnged in hot water, emit a 
strong and i)nngent smell of garlic, more or less intense according to 
the species and to the accidental circnmstances inllnencing it, the power 
cnlminating in Hiidltnid dUldrid. All Ilj/alinitv do not however 
secrete this nnpleasant odour, as at least one foreign species emits a 
fragrant musky perfume, and has been styled Ilydluud frdgrdns 
on this account. Certain species of CldHsiUa and Fupd have been 
observed to give ont a perceptible spermatic odour, as also does 
Aple.rd Inipnorum when the animal is crushed. Fleliv pomdtid is 
said to smell strongly of hemlock in the beginning of June, a smell 
which does not proceed from browsing on the plant, but from an 
exhalation peculiar to thesea.son of reju-oduction, while Fuio pictorum 
and allied s])ecies at the same season have been stated to strongly 
exhale a caprine or goat-like odour. 
The sense of Sight is possessed by the bulk of our native species, 
either in the larval or adult stage, but it is probable that all our 
species are, in addition, more or less acutely sensible to the iierception 
of light and shade, owing to the sensitiveness of the neuro-epithelial 
cellules of their external integument. 
It was formerly thought that the mollusca were (piite blind, the 
eye-specks being regarded as merely ornamental si)ots without any 
useful function, but their com})lex structure would a])pear to indicate 
greater visual power than they have as yet been actually shown to 
possess, and in the least S2)ecialized forms will at least enable the 
creatures to discriminate night from day and thus warn them to seek 
places of safety from their more numerous diurnal enemies. 
The visual organs in the various gvoups are known as Cei)halic, 
Dorsal or Pallial eyes, according as they are develoi)ed upon the head 
region or upon the more ex})Oseil portions of the body or mantle, 
their origin being probably due to the local moditication of the 
external integument through the attraction and accumulation of 
pigment gi’anules at those i)arts of the body most exposed to the 
action of light. 
The Cephalic Eyes originate behind the velum and in their primi- 
tive form are merely simjjle cup shaped invaginations of the external 
epithelium lined with pigmented and retinal cells and rods, such as 
are still possessed by Pdtelld and other archaic species. They were 
probably originally possessed by all mollusks, and are retained in 
