230 HISTORY OF THE ORMEE. 



discuss in detail the ormer's adaptations. In the limpet both 

 gills have disappeared, and new breathing organs have appeared 

 on the thin skirt which lines the inner surface of the shell's 

 rim. In other Gastropods, Trochus for example, the incoming 

 stream of the right side became weaker (for reasons too 

 involved for present discussion) and most of the breathing and, 

 finally, all of it was done by the gill of the left side, its fellow 

 on the right disappearing altogether. The outgoing current 

 then went out along the right side, and so no longer interfered 

 with that coming in along the left. The shell muscle of the 

 left side was to some extent an obstacle to the free entry of 

 water on that side, and it disappeared from Trochus and was 

 reduced in the ormer, in which latter also the process of 

 reduction of the right gill has begun. When the left shell 

 muscle was reduced the shell, unsupported on this side, sagged 

 somewhat ; and this enables us to see why it is on this side that 

 the shell of the ormer flattened itself down, and it also helps us 

 to understand the direction of the spiral curl of the shell of a 

 Trochus. 



The intrusion of grit and seaweed, and the difficulty of 

 breathing, must have been specially serious drawbacks to forms 

 living in such places as the ormer frequents. To overcome 

 the difficulty of breathing, variations in the direction of in- 

 creased length of the gills have survived, and as a natural 

 consequence, the cavity containing the gills has become very 

 deep. 



It has already been said that the ancestral type had a slit 

 in its shell rim so placed that it hastened the exit of the out- 

 going stream of water from the gill-cavity. Such a slit would, 

 however, have to be unusually long to be useful to the ormer 

 with its deep gill-cavity ; while any slit at all, much more a 

 deep one, would be a drawback in the peculiar conditions of 

 our type, which must be specially liable to intrusion of grit and 

 seaweed into the cavity. A very simple and effective modifi- 

 cation has survived and overcome this drawback. The two 

 edges of the slit have grown together except at certain fairly 

 regular intervals where, therefore, holes are found. These 

 form the well-known row of holes around the left side of an 

 ormer shell. Of these holes the old ones are continually being 

 closed, so that only the six latest added remain open. 



If an ormer be taken out of its shell, a thin flap of skin 

 (mantle) is seen all around. This mantle has the function of 

 forming the shell substance. Over the gill-cavity it is specially 

 thickened, and above the front end of the cavity a deep slit is 

 seen in its anterior edge. The slit therefore survives in the 



