i8o 



MOLL US KS. 



that form an operculum or valvular lid, so disposed as to shut up the aperture 

 when the animal retires, into its shell, but easily opened for the protrusion of 

 the cirri. The acorn-shells are widely spread : groups of different species are 

 found covering rocks, floating wood, shells of various kinds, or even the backs 

 of crabs and lobsters any objects, in short, which will afford them a secure 

 resting-place, 



FIG. 182. -ACORN-SHELLS ON THK 

 SHELL OF A WHELK. 



FIG. iSi. BARNACLES. 



CHAPTER XIV. 

 THIRD DIVISION OF INVERTEBRATE ANIMALS. 



MOLLUSKS, 



WIDELY different in their appearance from the Articulata 

 described in the last chapters are the creatures that next 

 present themselves. Instead of possessing an external skeleton, 

 divided into numerous segments and furnished with jointed limbs, 

 the Mollusks are either entirely naked and defenceless, or else 

 have their bodies protected by shells, in which they frequently 

 reside, and hence are generally known by the name of " shell- fish." 

 Formerly the animals which formed and inhabited these shells 

 were little known, and consequently little attended to. The shells 

 alone attracted the attention of the student, and hence the study 

 of this branch of natural science received the name of Conchology,* 

 and the appellation is still in general use. At the present day, 



, conche, a shell ; Ao-yos, logos, a discourse. 



