1914] Daniel: The Anatomy of Heterodontus francisci 161 



"While the variation in size of the secondary scales is due largely 

 to time of origin, other factors than priority evidently come into play 

 in the case of the spinules, as is shown by the fact that in the adult, 

 after the secondaries are fully matured, some of the spinules remain 

 as noticeably the largest scales of the pattern. 



2. Variations in Size Correlated with Function. — Both the embry- 

 onic spinules and the secondary scales show great differences in size 

 correlated with the position which they take on the body. For ex- 

 ample, an embryonic spinule in the region protected by the dorsal fin 

 is normally inferior in size to an embryonic spinule located on the 

 supraorbital crest. The same holds true also for the secondary scales 

 in the same areas. 



(a) Marginal scales as modified hy function. — The bearing of func- 

 tion on size is particularly well shown in marginal scales. In plate 9, 

 figure 8, the exposed marginals at the tip of the nose form an area of 

 relatively immense scales. Later, in the adult where the same area is 

 more greatly exposed ten to twelve rows of immense scales occur. 



A most interesting functional modification of the marginals is seen 

 in plate 8, figure 4. Here the marginals, even in an immature speci- 

 men, form a compact pavement of hypertrophied scales over a large 

 area. By what factor are these particular marginals so exposed as 

 to cause their early maturity? In order to understand this a knowl- 

 edge of the behavior of the young heterodont is indispensable. 



It may be said that both the young and the adult heterodont in an 

 aquarium remain the larger part of the time on the bottom. But, 

 while the adult lies flat on the bottom, the young has the interesting 

 habit of standing for considerable periods of time on its paired fins. 

 Now, from this position the paired fin becomes bent so as to press the 

 marginal area, together with a part of the ventral area, against the 

 floor. Under the stimulus thus produced the scales in this area reach 

 maturity at a time when the near-by dorsals and the more proximal 

 ventrals are sparse in numbers and relatively of insignificant size. 

 But that function in the individual is not wholly responsible for the 

 modification of this area is shown by the fact that a considerable 

 pavement is present before the young leaves the shell. 



That outward surroundings are able to influence the size or form 

 of a scale seems incredible when one considers that a scale is covered 

 with the hardest of organic substances, namely, enamel. That such 

 is the case is due to the fact that the scale just forming is exceedingly 

 plastic. On the surface of the skin the developing secondary scales 



