GASTEROPODA. 287 



fewer than Ancylus. The anomalous genus Amphihola (p. 139) 

 is said to have 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 Limncea or Flanorhis 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 withdrawn ; its teeth glitter like glass-paper, and in Limncea it 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 Yan Beneden and AVindischmann,f by Oscar 

 Schmidt,! and by Gegenbaur ;§ the memoir by the last-named 

 author, contains full information respecting Limax and ClausiHa, 

 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 velum of marine gasteropods — which in Doris, Polycera, and 

 uEolis, 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. 



t Eechenihes sur I'embryogenie des Limaces. Muller's Archiv. 1841. 

 X Ueber die Ent\\'ickeliing von Limax agrestis. Miiller's Archiv. 1851. 

 $ Beitrage zur Eatwickelungsgeschichte der Land-gasteropoden. Siebold and 

 KoUiker's Zeitschrift, 1862. 



