Growth-changes in Brittle Stars. 121 



4. The oral shields lie originally on the dorsal surface and only by subse- 



quent developmental changes come to lie on the ventral side. 

 The apparent exception shown by Aster onyx seems to me an un- 

 doubtedly secondary condition; this is indicated by their late and 

 unequal appearance. 



5. The adoral plates develop from the second pair of adambulacral 



(= side arm) plates. A large size relatively, as compared with 

 the oral shields, is a primitive condition, but a large size actually, 

 so that they are equal to or exceed the basal side arm-plates, is a 

 secondary and specialized condition. On the other hand, their 

 great reduction or entire absence is also a very specialized con- 

 dition.^ 



6. The jaws {oral plates) arise, in large part at least, from the first pair of 



adambulacral plates. Obviously the more clearly they show their 

 origin the more primitive is their character, while the moi'e com- 

 pletely they are fused and overshadowed by either the torus or 

 the adoral plates, the more specialized is their condition. 



7. The toriis arises, at the tip of the jaw, as a small plate not much higher 



than wide. A low, wide torus, therefore, indicates a primitive 

 condition, while a high, narrow one or a high one expanded at the 

 lower end indicates specialization. 



8. The teeth arise in association with the torus, but not as a part of it. 



The uppermost tooth of the adult was the first one developed, the 

 later teeth appearing below it successively. Obviously, a small 

 number of teeth is a primitive condition, and the same is true 

 where the lower teeth are evidently smaller than the uppermost. 

 Young teeth are sharply pointed ; therefore pointed teeth are evi- 

 dently more primitive than those which are rounded or truncate. 

 A large number of truncate teeth of equal size shows a decidedly 

 specialized condition. 



9. The dental papillce, where they occur, are homologous with the teeth, 



but are a modified form of the latter. Their occurrence is there- 

 fore undoubtedly a mark of specialization, the degree of which 

 may be indicated by their number and arrangement. 



10. The oral papilla arise in connection with the first and second adambu- 



lacral plates {i. e., the orals and the adorals) and appear to be 

 homologous with tentacle-scales. There is as yet no evidence to 

 determine whether their entire absence is a primitive or highly 

 specialized condition. Possibly it may be either. There is evi- 

 dence to show that at least a relatively primitive condition is 

 indicated when there is one papilla in connection with each oral 

 and each adoral plate. There can be scarcely any doubt that the 

 presence of more than one papilla in association with each of these 

 plates indicates specialization, the degree of which is suggested 

 by the number of such papillae. The primitive form of oral- 

 papillae appears to be scale-like, flat with rounded tip; pointed and 

 spine-like papillae are specialized forms. 



11. The genital-slits or bursal openings appear very early, one on each side 



of the base of the arm. The evidence is still lacking as to whether 



» In Mortensen's (1912) very valuable and interesting account of growth-changes in Asteronyx, he speaks 

 of the "seitenmundschilder" being resorbed and later as being greatly reduced. But examination of his 

 admirable figures shows there has been no resorption; the fact is that having reached a relatively large size 

 quickly, their later growth has been slower than that of the rest of the disk; thus in his specimen 1.9 mm. across 

 the disk, the adoral plates are each 0.55 mm. long, in the specimen 3.5 mm. across the disk, they are 0.60 mm. 

 long, and in the specimen 25 mm. across they are 3 mm. long. Surely there is no resorption here. 



