THE MOLLUSOAN EADULA. 133 



seems to say that the two structures keep pace in the younger 

 end of the radular sheath^ but that later the epithelium may be 

 delayed. This admission amounts to granting that the epi- 

 thelium throughout its length cannot keep pace with the 

 membrane except by stretching — if this word can be permitted 

 to stand for the conversion of the high columnar cells of young 

 basal epithelium into lower cells. For if equal increments of 

 epithelium and of membrane are added in the radular sac, then 

 clearly there will be no gliding of onestructureover the other 

 in any part of their length : they will keep pace with each 

 other. If, however, the increments of epithelium are less, then 

 cells of the epithelium must "stretch" or increase in volume 

 in order to keep pace with the overlying membrane. It is 

 highly probable that the increments of epithelium are less than 

 those of the membrane, for in the mouth-cavity the epithelium 

 is generally lower than in the sac; in Helix aspersa one cell 

 in the basal epithelium in the mouth-cavity covers as great a 

 length of membrane as four cells in the sac. Consequently if 

 we were to assume that equal lengths of epithelium and of 

 basal membrane were added in the sac, this would involve a 

 considerable relative movementof the epithelium and membrane 

 in the mouth-cavity, the epithelium moving inore quickly than 

 the membrane, and this is wholly unlikely. 



That the odontoblasts are replaced by fresh cells derived 

 from the cell aggregate at the extremity of the radular sac 

 seems most probable and is, I think, the view which has gained 

 acceptance; at the same time, investigators differ among them- 

 selves as to whether the replacement is gradual, so that each 

 group of odontoblasts secretes several teeth before it passes on 

 into the basal epithelium, or, more sudden, each odontoblast 

 group only secreting once before it is relieved by recruits. 

 On full consideration this view will be found to involve 

 a somewhat remarkable life-history for the odontoblasts. 

 Starting from the indifferent cell mass from which they arise 

 by cell-division, they become elongated and form a set of cells 

 which possess as a whole a definite and constant shape. They 

 secrete chitin first for the teeth, next for the basal membrane 



