broad, wlnle those of animals livin^j; nearer (lie low tide 

 limits and in sheltered positions are often high and 

 narrow, but 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 ehiefly by additions to its posterior border 

 and its mouth is thus lengthened and broadened until it 

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

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

 closed by secretion of nacreous material on the inside. 

 In further groAvth the anterior border seems to have a 

 larger share and older shells thus usually have the a})ex 

 further back than young ones. This, however, depends 

 larg-ely 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 animal's 

 special mode of life. Limpets are peculiarly exposed to 

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

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

 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 aud tide; at the same time the shell is an 

 ettective protection against enemies, for it can be pulled 

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

 marijin of the scar increases the dihicultv of detachment. 

 The peripheral position of the shell-muscle, and its 

 symmetrical disposition are adaptations to this protective 

 airangenient. 



The spiral-shelled (iastropods protect themselves by 



