266 



ZOOLOGY. 



showing the lung and heart (/«) .ind the mouth (»*) as well 

 as the four tentacles, with an eye at the end of the two 

 upper tentacles. Fig. 203 shows the 

 brain and pedal ganglia of Helix alhola- 

 Iris. The tentacles when carefully exam- 

 ined may be found to contain both the 

 eyes (<?) with the optic nerve {op) and the 

 olfactory nerve '(Fig. 206, o). Fig. 204 

 represents the jaw and lingual ribbon of 

 Helix. 



Tlie eggs of the pond-snails are laid in 

 transparent capsules attached to sub- 

 merged leaves, etc. Those of Physa 

 heterostropha are laid in the early spring, 

 and three or four weeks later from fifty to 

 sixty embryos with well-formed shells may 

 mgAm.— Limn cats ap- be fouud in the caj)sule. 



presms.—Pdt&v Morse. rT\^ £ t t ° n i i_ • 



ihe eggs ot Lymnceus are laid late m 

 tlie spring in capsules containing one or two eggs, and sur- 

 rounded by a mass of jelly. After passing through the mo- 



FifT. 200. — Lymnceus elodes^ a common pond-snail, showing its variations. — 

 After Morse. 



rula, gastrula, and trocliosphere stages a definite veliger 

 stage is finally attained. The foot is large and bilobed, the 

 mantle and shell then arise, and the definite molluscan char- 

 acters are assumed, the shell, creeping foot, mantle-flap, eyes, 

 and tentacles appearing, and the snail hatching in about 

 twenty days after develoiDment 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. 



