328 OBSERVATIONS ON MOLLUSKS. 



arrived at their full growth ; and even some individuals 

 which are themselves so immature as to possess 

 hardly any of the distinguishing characters of the 

 species to which they belong, frequently contain 

 young of a sufficient size to be seen from without 

 through the transparent valves. 



ferous gasteropoda can sustain themselves on the surface of the 

 liquid by taking in a larger quantity of air. But this explanation 

 in no way applies to those gasteropods which breathe by gills, and 

 still less perhaps to those which, like the Actsson for example, 

 possess no appendage ; and all these mollusks exhibit the same 

 phenomenon. I have even seen a small Actseon move itself in 

 exactly the same manner in the very body of the liquid. These 

 facts find a very simple explanation in the existence of vibratory 

 cilia ; and this mode of locomotion is not more extraordinary in 

 the case of these mollusks than it is in that of the Infusoria, that 

 of the Planarite, the Nemertes, &c." 



There is no allusion, by M. Quatrefages, to the circumstance 

 of any of the acephalous mollusks possessing this property, as well 

 us the gasteropods. 



