246 ZOOLOGY. 
showing the lung and heart (4) and the mouth (7) as well 
as the four tentacles, with an eye at the end of the two 
upper tentacles. Fig. 198 shows the brain 
and pedal ganglia of Helix albolabris. The 
tentacles when carefully examined may be 
found to contain both the eyes (¢) with the 
optic nerve (op) and the olfactory nerve 
(Fig. 201, 0). Fig. 199 represents the jaw 
and lingual ribbon of Helix. 
Fig. 191—Undersidag De eggs of the pond-snails are laid in 
of head of pond-snail. transparent capsules attached to submerged 
ic, mouth open show- 
ing the buccal cavity; leaves, etc. ‘Those of Physa heterostropha 
J, jaw; Yj, lateralteeth; eer . 
7, lingual ribbon; ¢, are laid in the early spring, and three or 
‘scam four weeks later from fifty to sixty embryos 
with well-formed shells may be found in the eapsule. 
The eggs of Zimneus are laid late in the spring in 
capsules containing one or two eggs, and surrounded by a 
mass of jelly. After passing through the morula, gastrula, 
Fig. 195.—Mouth-parts of the Fig. 195a.—Sea-snail (Sycotypus) bor- 
LaPeer protruded. ¢, tongue; ing into a shell. 4, mouth (m) ap ee 
j, lateral teeth; j, jaw; r, rasp, or the rasp (7) retracted; B, mouth pressed 
dingual ribbon. against a shell, , the rasp gliding over 
a tendon like a pulley, and filing a hole 
ants the shell; the arrow points into the 
roat. : 
and trochosphere stages a definite vel?zer stage is. finally 
attained. The foot is large and bilolled, the mantle and 
‘ghell then arise, and the definite molluscan characters are 
assumed, the shell, creeping foot, mantle-flap, eyes, and 
tentacles appearing, and the snail hatching in about twenty 
days after development begins. 
Land-snails and slugs lay their eggs loose under damp 
leaves and stones, and development is direct, the young 
snail hatching in the form of the adult. 
