88 GASTROPODA. 



The organs of touch are one or two pairs of con- 

 tractile tentacles, placed commonly on the back of 

 the head. In some cases these can be inverted 

 and everted; but more generally they are solid. 

 The hinder pair, where there are two, are often 

 more complex in structure than the others, and 

 are, perhaps, the seat of the sense of smell. 



Well-developed eyes are almost invariably present 

 in this Class. They never exceed a single pair, 

 and are generally placed on some part of the ten- 

 tacles, the hinder ones, when there are two pairs. 

 The most common position for these organs is at 

 the extremity of a short fleshy column, springing 

 from the base of the tentacle. In many species 

 the eyes present an elaborate structure ; the great 

 Strombida of the tropical seas, for example, have 

 eyes with " a distinct pupil and a double iris, 

 equalling, in beauty and correctness of outline, 

 those of birds and reptiles;"* and many of our 

 native genera, as Buccinum, Fusus, and Murex, are 

 scarcely inferior, in this respect, to the Strombidce. 



The organs of hearing are as rudimentary as in 

 the CONCHIFERA. They are variously placed in 

 different genera, but most commonly on the upper 

 side of the great brain-ganglion, near the eyes (as 

 in the NudibrancJiid) , or on the lower side of the 

 neck, and consist of a pair of transparent capsules, 

 filled with a clear fluid, in which are suspended 

 one or more glassy bodies, called otolithes (ear- 

 stones). The number of these bodies sometimes 

 reaches to eighty in each capsule. Their move- 

 ments " are even more marked than in the CON- 

 CHIFERA ; and the balancing and rotating of each, 



* Guilding. 



