THE SENSORY RESPONSES OF CHITON 165 



project beyond the posterior margin of the valve. When the 

 chiton reaches a length of 1.5 cm. the beak begins to project, 

 when 5 and 6 cm. in length, the umbo of the preceding valve 

 (third), and of the other valves, becomes noticeably eroded. 

 Correlated with the incipient exposure of the intertegmental 

 mantel between valves three and four — produced by the erosion 

 of the beak of valve three — there occurs a growth of the anterior 

 edge of valve four, forming a protecting projection into the 

 laminal sinus of this valve, which compensates for the wearing 

 down of the beak on valve three. These relations are illustrated 

 in figure 3.^ We do not consider here the fine question as to 

 whether this compensating growth of the tegmentum along the 

 anterior margin of the valve is a normal growth process, inde- 

 pendent of erosion. Valve four was selected for measurement 

 partly because of its intermediate position in the series of shell 

 plates, and hence its possibly greater freedom from the opera- 

 tion of inherent growth tendencies of this character, but more 

 particularly because, being the shortest of the anterior valves, 

 slight erosional or other changes would produce greater percent- 

 age alterations in its length, which would thus be more 

 easily detected. All that we wish to show here is, that the 

 'erosion point' — i.e., the average length at which the chiton in- 

 dividuals in a particular locality exhibit the effects of weathering 

 upon the valves — can be determined and exhibited in a non-sub- 

 jective manner. The irregularities in the relation between valve 

 length and length of individual are due to the concomitant 

 operation of these two tendencies, erosion at the beak and for- 

 ward growth at the anterior mid-point of the valve, superimposed 

 upon the normal growth of this structure. 



Accompanying the erosion of the valves, which tends to pro- 

 duce upon their surface a chalky, brownish, or pale slaty hue, 

 there occurs a plentiful accumulation of adventitious organisms 

 upon the dorsal surface of Chiton (Plate, 'Ola, pp. 380 ff.). The 



^ These matters are touched upon at this point for a reason which will become 

 apparent further on in this paper. They are discussed with some completeness 

 in a subsequent paper, by one of the present writers, dealing with the ethology 

 of chiton. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 2 



