GASTEROPODA. 287 
fewer than Ancylus. The anomalous genus Amphibola (p. 189) 
is said to haye a tongue, armed with teeth similar to those 
of the slug. 
About one-third the lingual membrane is spread over the 
tongue; the rest has its margins rolled together, and is lodged 
in a sac or dental canal, which diverges downwards from the 
posterior part of the mouth, and terminates outside the buccal 
mass of muscles.* 
The mode in which the tongue is used, may be seen by placing 
a Limnea or Planorbis in a glass of water, inside which the green 
conferva has begun to grow; they will be observed incessantly 
cleaning off this film. The upper lip with its mandible is raised, 
the lower lip—which is horse-shoe shaped—expands, the tongue 
is protruded and applied to the surface for an instant, and then 
withdrawn ; its teeth glitter like glass-paper, and in Limncea it 
is so flexible, that frequently it will catch against projecting 
points, and be drawn out of shape slightly as it vibrates over 
the surface. 
“‘The development of the (in-operculate) Pulmonifera has 
been worked out by Van Beneden and Windischmann,t+ by Oscar 
Schmidt,t and by Gegenbaur ;§ the memoir by the last-named 
author, contains full information respecting Limawz and Clausilia, 
and some important notices with regard to Helix. 
‘The yelk undergoes complete division. The first stage of 
development consists in the separation of the embryo into 
mantle and foot. The anterior part of the body, in front of the 
mantle, dilates and forms a contractile sac—the homologue of 
the velwm of marine gasteropods—which in Doris, Polycera, and 
Aolis, has been seen to exhibit similar contractions. (Gegen- 
baur.) To this contractile vesicle the name of Yelk-sac was 
given by Van Beneden and Windischmann, but it is a very 
different organ from the true Yelk-sac, which exists in the 
Cephalopoda alone among molluscs. 
‘* A similar contractile dilatation exists at the end of the foot 
—and the contractions of this ‘caudal’ vesicle and of the 
‘ vitellary ’ vesicle alternate, so as to produce a kind of circula- 
tion before the development of the heart. 
‘The oral tentacles and parts about the mouth are the last to 
be completed. 
** A peculiar gland exists during the embryonic period, at- 
* Thomson, An. Nat. Hist. Feb. 1851. 
} Recherches sur l’embryogenie des Limaces. Miiller’s Archiv. 1841. 
t Ueber die Entwickelung von Limaz agrestis. Mliiller’s Archiv. 1851. 
§ Beitrige zur Entwickelungsgeschichte der Land-gasteropoden, Siebold and. 
KGiliker’s Zeitschrift, 1852. 
