THE SNAILS, OR HELICID&. 



333 



degrees rubbed tolerably smooth in some places and encrusted in others with corallines, 

 calcareous matter, and the shelly coatings of various marine zoophytes. Sometimes the 

 sea-weeds find a lodgment on the shell, as is often the case with other comparatively 

 stationary mollusks, such as the common limpet ; and in that case the algae not only find 

 a home, but conceal their protector by their waving fronds. 



The accompanying illustration represents the MARBLED CHITON, a rather prettily colored 

 shell, its exterior being rusty-red mixed with brown and yellow, and edged with brown. The 

 SHORT-SPINED CHITON is covered 

 with short spines. Its color is 

 sooty -black, but this dull uniform- 

 ity of a sombre hue is more than 

 redeemed by the beautiful and 

 minute pencilling with which its 

 surface is engraved. The BANDED 

 CHITON, or CUITONELLA, has been 

 removed by modem naturalists into 

 a separate genus, on account of 

 the formation of the armor. The 

 plates do not cover the entire sur- 

 face as in the preceding genus, as 

 only a portion is seen above the 

 mantle. The defence is, however, 

 nearly as perfect as in the previ- 

 ous genus, as the projections ap- 

 proach each other beneath the surface of the mantle, and would act as effectually in shielding 

 the internal organs as if the plates had met on the surface. These creatures are generally 

 found in the clefts of canal rocks. 



The animal is more active than the limpet, but does not appear to be very locomotive 

 in its habits. Its broad creeping disc adheres very strongly to the rocks, and holds the animal 

 so firmly that, if it should happen to have taken up its abode within a crevice, to extract it 

 without tools would be an impracticable task. Like the dentalium, this creature possesses 

 neither eyes nor tentacles. The figures in our illustration are of natural size. 



MAKBLED CHITON. - Chiton marmoreus. 



INOPERCULATE AND OPERCULATE GASTEROPODS. 



PASSING from the sea to the land, we come to those gasteropods which breathe atmospheric 

 air, and ai'e furnished with respiratory organs suited to the lower element in which they live. 

 These creatures fall naturally into two large sections, the one being destitute of an operculum 

 and the other possessing that remarkable appendage. They are respectively called inoper- 

 culate and operculate gasteropods, and it is with the former that we have now to deal. The 

 inoperculate are generally furnished with large shells ; but in some, such as the slugs, the 

 shell is either very small or wholly absent. The shell of these animals, when present, is not 

 nearly so hard and porcelain-like as that of the sea-snails, and contains a much larger propor- 

 tionate amount of animal matter. It is worthy of notice, that in order to prevent the waste of 

 moisture in those species which live on land, and the entrance of water in those which inhabit 

 the ponds and rivers, the respiratory passage is small, and closed with a kind of valve. 



This group embraces the largest number of species of mollusks, including snails, slugs, 

 whilks, limpets, couries, etc. The head is well developed, and one or two pairs of tentacles 

 are present. The sexes are usually separate. 



THE first family is that of the Snails, or Helicida3, containing a vast number of species. 

 Most of the Snails have a shell large enough to permit the animal to withdraw itself wholly 

 into the protecting domicile. During the time when they are active these creatures require no 



