THE PTEROPODS. 159 



strong teeth, placed in transverse rows, and arranged in three 

 series ; each central group consists of four spines, while those 

 on the sides contain but two apiece. In the upper part of the 

 circumference of the mouth, we find a semi-circular, horny plate, 

 resembling an upper jaw, and the tongue, by triturating the food 

 against this, gradually reduces substances, however hard. On 

 opening the limpet the tongue is fodhd doubled upon itself, and 

 folded in a spiral manner beneath the viscera. 



Although they are deprived of all higher instincts, yet we find 

 among the gasteropoda a few examples of concealment under 

 extraneous objects, which remind us of the masks and artifices 

 frequently employed by the insects and the crustaceans. The 

 agglutinating top (trochus agglutinans) covers itself with small 

 stones and fragments of shells, and, thus shielded from the view, 

 escapes the voracity of many an enemy but little suspecting the 

 savoury morsel hidden under the little mound of rubbish which he 

 disdainfully passes by. In animals whic^ are only provided with 

 passive means of defence, we may naturally expect a consider- 

 able degree of caution, and in this respect the gasteropoda 

 might give many useful lessons to man. See how carefully our 

 garden-snail protrudes her tentacles, as far as possible, to sound 

 every obstacle in her way before she drags herself farther 

 along, and how quickly she withdraws into her shell at the 

 least symptom of danger. What an example to so many of 

 us that leap before they look, and frequently break their necks 

 in the fall. 



While the snails, whether terrestrial or marine, are proverbi- 

 ally slow, the pteropods, or wing-footers, swimming by the aid 

 of two muscular expansions resembling fins, and attached to 

 the opposite sides of the neck, are remarkable for the velocity 

 of their movements. They have no disk to walk upon like 

 the gasteropods, but resemble them by the 

 possession of a head distinct from the rest 

 of the body, which some, like the hyaleas 

 and cleodoras, conceal in a thin trans- 

 parent or translucent shell; while others, 

 like the blue and violet clios, beautifully 

 variegated with light red spots, are per- 

 fectly naked. They are met with in all 

 seas under the equator, as well as in the vicinity of the polar 



