PATELLA. 199 



broad, wliile those of animals living nearer tlie low tide 

 limits and in sheltered positions are often high and 

 narrow, bnt these rules are not without exceptions. 

 Plate I., fig. 1, shows extreme types of shell. 



The larval shell shows the beginning of a spiral. This 

 shell grows chiefly by additions to its posterior border 

 and its month is thus lengthened and broadened until it 

 has a cap-shape with the apex (the remains of the spiral) 

 far forwards. This apex then breaks off and the hole is 

 closed by secretion of nacreous material on the inside. 

 In further growth the anterior border seems to have a 

 larger share and older shells thus usually have the apex 

 further back than young ones. This, however, depends 

 largely on the precise form of the shell, the broad low 

 cones above mentioned having the apex further back than 

 the tall narrow ones. The transition from spiral to 

 conical shell, with the correlated consolidation of the 

 visceral hump and the broadening of the foot into an 

 oval adhesive sole, are all adaptations to this animals 

 special mode of life. Limpets are peculiarly exposed to 

 the wash of the tide and of storms, and it is an advantage 

 to them to be able to adhere firmly to the rock and 

 present as small an area as possible to the waves. Hence 

 the thick, plate-like foot and the preparation, of a 

 smooth surface to which it can effectively cling. The 

 shape of the shell offers a minimum amount of resistance 

 to waves and tide ; at the same time the- shell is an 

 effective protection against enemies, for it can he pulled 

 down so as to completely cover the animal, while the deep 

 margin of the scar increases the difficulty of detachment. 

 The peripheral position of the shell-muscle, and its 

 symmetrical disposition are adaptations to this protective 

 arrangement. 



The spiral- shelled Gastropods protect themselves by 



