46 OCEAN LIFE. 



their waving motion, as swimming fins. The proper function of 

 the mantle-edge is, however, the formation and increase of the 

 shell ; and hence it is always found, in shell-covered species, on 

 that part of the body which is near the orifice. In a few species, 

 the shell is included within the substance of the mantle. A dis- 

 tinct head is always present, more or less. conspicuous according 

 to the degree in which it- projects from beneath the front of the 

 mantle. It is furnished with various organs of sense. The organs 

 of touch are one or two pairs of contractile tentacles, placed com- 

 monly 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 tentacles, the hinder ones, when there 

 are two pairs. The most common position for these organs is at 

 the extremity of a short fieshy column, springing from the base 

 of the tentacle. In many species the eyes present an elaborate 

 structure ; the great Strombidce of the tropical seas, for example, 

 have eyes with l 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 Bucsinum and Murex, are 

 scarcely inferior, in this respect, to the Strombidce. 



" There is always in these animals a distinct mouth, placed at 

 the front of the head, furnished with swollen, contractile lips, 

 often prolonged into a cylindrical proboscis. In many species, 

 as in Eolis, Tritonia, $c., there are two horny, sharp-edged 

 plates, which act as jaws, playing over each other, like the 

 blades of shears. Sometimes there is only a single jaw-plate 

 inserted in the palate. There is, beside the jaws, a fleshy band, 

 longer or shorter according to the genus, which performs the 

 function of a tongue. It is sometimes grooved along its surface, 

 and is always armed with horny teeth or spines, which are ar- 

 ranged in regular rows, both longitudinally and transversely. 

 The form, number, and pattern of these teeth vary greatly, 

 though always constant in the same species. They ' are amber- 

 colored, glassy, and translucent ; and being siliceous (they are in- 



