CLASS XIII. 
MOLLUSCS (MOLLUSC4). 
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 anima!: have a shell, others are naked. Most of 
them live in water, but some on land, wich 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 ifference more or less in 
other classes also of invertebrate animals, nay even in the last class 
of vertebrates, that of fishes}. 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’. 
Above the tongue is a transverse horny plate with projecting lines 
1 Here let it suffice to point to Myxine, and especially Amphioxus ; comp. also what 
we said above, pp. 34, 35- 
2 Compare on this subject F. H. Troscuen Veber die Mund-theile einheimischer 
Schnecken, WIEGMANN’S Archiv, 1836, Bd. 1. s. 257—279, Taf. 1x. X., and especially 
Lovin in Ofversigt af Kongl. Vetensk-Akademiens Férhandlingar, 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 Zolis. Ann. of Nat. Hist. 
Xv. 1845, pp. 9, IO. 
