GASTEROPODA. 161 



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 Limn&a 

 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 and then withdrawn ; its teeth glitter like glass-paper, and in Limnaa it is 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,f by Oscar Schmidt,! and by Gegen- 

 ur^ the memoir, by the last named author, contains fall information 

 respecting Limax 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 velum of marine gasteropoda which in Doris , Poly- 

 cera, and JEolis, 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 circulation before the development of the heart. 



" The oral tentacles and parts about the mouth are the last to be com- 

 pleted. 



" A peculiar gland exists during the embryonic period, attached to the 

 parietes of the ' vitellary' vesicle, which Gegenbaur and Schmidt compare to 

 a Wolffian body. 



" Gegenbaur draws attention to the fact, that the first rudiment of the 

 shell in Limax, Clausilia and probably Helix, is not secreted on the exterior 

 of the mantle, as in other gasteropoda ; but is deposited, in the form o 

 calcarious granules, within its substance. 



" Besides, therefore, the possession of Wolffian bodies, and of especial 

 contractile organs, which subserve respiration and circulation during em- 

 bryonic life the terrestrial, gasteropoda are further distinguished by the 



* Thomson, An. Nat. Hist. Feb. 1851. 



f Recherches sur I'embryogeme des Limaces. Miiller's Archiv. 1841. 

 t Ueber die Entwickelung von Limax agrestis Miiller's Archiv, 1851. 

 Beitrage zur Entwickelungs geschichte der Land-gasteropoden. Siebold and 

 Kblliker'* Zeitschrift, 1852. 



I 2 



