422 



GASTEROPODA. 



a very complete defence against the assaults of foes that might 

 attack them while they are concealed in their habitations, and, in 

 such a posture, necessarily helpless and incapable of resistance. 

 The provision for their protection is sufficiently simple : attached 

 to the posterior extremity of the body, which is the part last drawn 

 into its abode, is a broad horny or calcareous plate (fig- 196, g), 

 called the operculum ; this is of variable dimensions in different 

 species, but always in shape accurately corresponding with the 

 contour of the mouth of the shell. By this elegant contrivance 

 a door is closely fitted to the aperture of its retreat whenever the 

 mollusk retracts itself within its citadel; and, thus defended, it 

 may safely defy external violence of any ordinary description. 



(463.) A most remarkable exception to the usual univalve con- 

 dition of the shells in the GASTEROPODA, is observable in one 

 solitary genus belonging to the Cyclobranchiate order. In Chi- 

 ton (fig. 197) we find, instead of a turbinated or shield-like 



Fig. 197. 



covering formed of one piece, a kind of armour composed of 

 several distinct plates, arranged in a longitudinal series along the 

 centre of the back, and overlapping each other like the tiles of 

 a house. 



In these curious animals the whole back is invested with a 



