74 



FIRST LESSONS IN ZOOLOGY. 



and the pond-snails only two. The eyes of the land-snails 

 are situated at the end of the tentacles; in the sea-snails 

 the eyes are not on the end, but on the head near the base 

 of the tentacles, and some kinds have nc eyes. Indeed, 

 some snails have no shells. In the slug (Fig. 74), which 



FIG. 76. Under side of head of pond-snail, be. mouth open showing the buccal 

 cavity; in the left-hand figure the mouth (HI) is closed by the lip (I); j, jaw; Ij, 

 lateral teeth; r, lingual ribbon; f, tentacle. 



is common under stones and sticks on land, the shell is 

 only a thin scale on the back, and covered by the skin. If 

 they climb a twig or spear of grass, they let themselves 

 down by a slimy thread. In fact, the body is covered with 



FIG. 77. Mouth-parts of the 

 pond - snail protruded, t, 

 tongue; //. lateral teeth; j, 

 jaw ; r, rasp, or lingual rib- 

 bon. 



FIG. 78. Sea - snail (Sycotypus) 

 boring into a shell. A , mouth 

 (m) at rest, the rasp (r) retract- 

 ed; B, mouth pressed against 

 a shell, 7-, the rasp gliding over 

 e tendon like a pulley, and fil- 

 ing a hole into the shell; the 

 arrow points into the throat. 



slime, and where a snail walks it leaves behind it a glisten- 

 ing slimy track, which soon dries. 



One can easily see how pond-snails eat, by watching them 

 in a glass vessel of water, as they move about either on 

 their backs at the surface of the water, or glide up the 



