246 



ZOOLOGY. 



showing the lung aud heart (h) and the mouth (m) 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 alMabris. The 

 tentacles when carefully examined may be 

 found to contain both the eyes (e) with the 

 optic nerve (op) and the olfactory nerve 

 (Fig. 201, o). Fig. 199 represents the jaw 

 and lingual ribbon of Helix. 



The eggs of the pond-snails are laid in 



Fig. 191. Underside . .. . , 



of head of pond-snail, transparent capsules attached to submerged 



be, mouth open show- , rl i, , , . , 



in ? the buccai cavity; leaves, etc. Those of Physa Jieterostrop/ia 

 ^^ih.guai^ri'bllonf *! are laid in the early spring, and three or 

 four weeks later from fifty to sixty embryos 

 with well-formed shells may be found in the capsule. 



The eggs of Limnmis 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- 



pond-snail protruded, t, tongue; ing into a shell. .4, mouth (m) at rest, 



ij, lateral teeth; j, jaw; r, rasp, or the rasp (>) retracted ; B, mouth pressed 



lingual ribbon. against a shell, , the rasp gliding over 



a tendon like a pulley, and filing a hole 



into the shell; the arrow points into the 



throat. 



and trochosphere stages a definite veliger stage is finally 

 attained. The foot is large and bilobed, the mantle and 

 shell 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 twent 

 days after development begins. 



Land-snails and slugs lay their eggs loose under dam 

 leaves and stones, and development is direct, the younj 

 snail hatching in the form of the adult. 



