§ 13. 



FOSSIL SHELLS. 217 



reous spar. Molluscous animals* are divided into mollusca, 

 properly so called, which are covered with a single shell, 

 as snails, periwinkles, &c. ; and conchifera^ having a shell 

 with two valves, as oysters, scallops, &c. The former are 

 of a higher organization than the latter, having eyes, and a 

 distinct nervous system ; the latter have neither eyes nor 

 head, and are therefore called acephala.\ Some genera are 

 herbivorous, living exclusively on vegetables ; others are 

 carnivorous ; and many have a retractile proboscis, fur- 

 nished with a rasp, by which they can perforate wood, 

 shells, stone, and other substances. The shells of the carni- 

 vorous testacea are also generally provided with a channelled 

 or grooved beak, for the reception of the fleshy syphon by 

 which the sea water is conveyed to the respiratory organs 

 (Llgn. 38, fig. 3, 4, 5) ; while the herbivorous have the 

 opening of the shell entire (Lign. 39, fig. 3, 5, 6, 7). Some 

 tribes are exclusively marine, others can only live in fresh- 

 water, while many are restricted to the brackish waters of 

 estuaries. Their geographical distribution is alike various : 

 certain highly organized forms (the Cephalopoda) inhabit 

 deep waters only, and are provided with an apparatus by 

 which they can rise to the surface ; while others are littoral, 

 that is, live in the shallows along the sea coasts ; many 

 exist in quiet, others in turbulent waters ; some are grega- 

 rious, like the oyster, while others occur singly or in small 

 groups. All these varieties of condition are more or less 

 strongly impressed on their shelly coverings, which may be 

 regarded as their external skeletons ; and the experienced 

 conchologist is generally enabled, by the peculiar characters 

 of the shell, at once to determine the economy and habits of 

 the animal, and consequently the physical conditions in 



Soft-bodied animals. f Shell-bearing animals. 



Having no head. 



Medals of Creation, vol. i. chap. 12, p. 363. 



