522 



GASTEROPODA. 



(1373.) Several of the Pectinibranchiate genera are provided with 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. 262, #), called the operculum; this 

 is of variable dimensions in different species, but always accurately cor- 

 responding in shape 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 de- 

 scription. 



(1374.) 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 Chiton (fig. 263) 



Fig. 263. 



Chiton : A, ventral ; B, dorsal aspect. 



we find, instead of a turbinated or shield-like 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. 



(1375.) In these curious animals the whole back is invested with a 

 dense leathery mantle of an oval form, and considerably more extensive 

 than the cavity containing the viscera. Where not covered by the 



