ON MOLLUSCA. 



211 



bend up its large foot and withdraw it completely into the 

 shell like other univalves ; even when it is plunged at once 

 alive into spirit, the foot remains rigid and almost straight. 

 Thus an operculum placed on the hinder surface of the foot, as 

 in Neritina, would be useless, since it cannot serve to close the 

 opening and to protect the soft parts, if the foot cannot be 

 withdrawn; but the Navicella, as has been said, does. not 

 need any such protection, since it clings more closely to the 

 rock when threatened with danger, and we might expect to 



a 



Fig. 61.— Diagram of section through Nerilina and Navicella to show the position oi: 

 the operculum in each genua, a, Mavkella (op, operculum), 6, operculum of ^^avi- 

 celta ; c, Nerilina {op, operculum), d, operculum of Nerilina. 



find, on examination, that it had no operculum. In this, how- 

 ever, we should be quite mistaken ; in all the Navicellse, without 

 exception, there is a true operculum placed, exactly as in all the 

 Operculata, on the back or upper side of the foot. It agrees 

 too in structure with that of Neritina, but it is much smaller 

 than the mouth of the shell (fig. 61), and the little hook 

 which server to attach it to the muscle of the foot, and 



