CLASS XIII. 

 MOLLUSCS (MOLL USC A). 



ALTHOUGH the name Molluscs has a more general signification, 

 and is also so used by us in this work, yet we prefer employing it 

 in a more limited sense for a class of animals which, in the former 

 edition of this handbook, we named cephalophorous molluscs. 

 Names of classes ought in our judgment to be short and not to be 

 confounded with definitions or descriptions. LAMARCK too has 

 already given the name of molluscs in the same sense to this divi- 

 sion of the animal kingdom. 



The animals of this class have a head more or less distinct 

 from the rest of the body. This head usually contains special 

 organs of sense for touch and vision, sometimes even for hearing. 

 Many of these animals have a shell, others are naked. Most of 

 them live in water, but some on land, which in the two classes 

 immediately preceding is never the case. However there are 

 amongst these animals very different degrees of perfection in the 

 organisation, yet we observe the same difference more or less in 

 other classes also of invertebrate animals, nay even in the last class 

 of vertebrates, that of fishes 1 . Accordingly we must here trace the 

 principal differences in the arrangement of organs according to the 

 natural groups and families. 



The oral cavity of molluscs forms a very muscular expansion, 

 at the base of which lies an organ, usually named tongue, which is 

 covered by little teeth or hooks placed in transverse rows. In some 

 this tongue is short and broad, in others ribbon-shaped and long 2 . 

 Above the tongue is a transverse horny plate with projecting lines 



1 Here let it suffice to point to Mijxine, and especially Amphioxus; comp. also what 

 we said above, pp. 34, 35. 



2 Compare on this subject F. H. TROSCHEL Ueler die Mund-theile einheimischer 

 SchnecJcen, WIEGMANN'S ArcJiiv, 1836, Bd. I. s. 257279, Taf. IX. x., and especially 

 LOVEN in Ofversigt af Kongl. Vetensk-ATcademiens ForJiandlingar, 1847, pp. 175 199, 

 who has described and figured these teeth in very many genera. That these hard parts 

 consist of silex was observed by HANCOCK and EMBLETON in Eolis. Ann. of Nat. Hist. 

 xv. 1845, pp. 9, 10. 



