VISUAL ORGANS OF MOLLUSCA. 353 



that the tentacles, when functioning as olfactory organs, have just 

 the same relations as the respiratory ones ; and with this we may 

 correlate the great distance backwards at which the tentacles are 

 sometimes found. 



In the Cephalopoda the olfactory organs are more definite in 

 character. Behind the eyes we find two small pits, or papilla^, which 

 are level with the surface ; these are ciliated. The processes of the 

 more deeply-placed olfactory cells stand up between the ciliated 

 ones ; they are supplied by a nerve which arises close to the optic 

 nerve. 



Visual Organs. 

 § 272. 



Visual organs are found in all Mollusca that are endowed with 

 the power of active movement. On the other hand they are always 

 atrophied in the fixed forms, although they are present during the 

 larval stages. This is also the case in the Placophora, where the 

 larvae indicate by the possession of a pair of pigment spots that they 

 are provided with eyes, which are atrophied later on. 



Structures of this kind are found on the nerve-centre, and in the 

 head of the Lamellibranchiata during the larval stages; they are 

 even provided with a refractive body, but they undergo degeneration 

 later on. The same thing happens in the Scaphopoda. 



It is different with the organs, which are ordinarily found on 

 the edge of the mantle, in the higher divisions of many Lamellibran- 

 chiata. They are carried on special optic-stalks (Area, Pectunculus, 

 Tellina, Pinna, etc.) ; in many cases (Pecten, Spondylus) they were 

 even observed by the older investigators, owing to the emerald- 

 green colour of the tapetum, which is found at the base of the eye. 

 Although these organs have many peculiarities of structure, they 

 agree in all essential points with the optic organs of the rest of the 

 Mollusca. They are supplied with nerves by the small trunks which 

 pass along the edge of the mantle. These organs vary greatly in 

 the extent to which they are developed, and are sometimes replaced 

 by mere pigment spots. This arrangement must be regarded from 

 that point of view, on which we have already insisted, when we 

 pointed out that sensory organs might be differentiated from simple 

 nerve-endings at any point of the integument ; these eyes, therefore, 

 on the edge of the mantle, can only be compared with the optic 

 organs which are found on the head, from a physiological point of 

 view ; morphologically, they are special structures, and are, like the 

 similar organs found in Vermes, due to adaptation. 



We never find more than one pair of cephalic eyes in the Gas- 

 tropoda. They are often replaced by mere spots, placed on the 

 supra-cesophageal ganglion, and disappear altogether when the 

 animal loses its power of free movement (Vermetus). "When it is 



2 A 



