478 Mr. D. W. Devanesen. Development of the Calcareous 



8. Discussion. 



In the mouth-fraine of recent Asteroidea as well as in most of the fossil 

 star-fishes are seen five pairs of generally triangular ossicles situated in the 

 inter-radial angles bordering the mouth (Spencer, 1913, p. 26). Originally 

 identified by Luclwig as the first pair of modified adambulaorals, they are now 

 known as mouth-angle plates. By their position both in the embryo and in 

 the adult and by their relation to the mouth, the five pairs of " alveoli " of the 

 lantern of Aristotle recall the five pairs of mouth-angle plates in star-fishes. 



In the section on " epiphyses," we saw how the two epiphyses each situated 

 on an alveolus belonging to adjacent jaws constituted a pair, and not the two 

 epiphyses which formed an arch. As the radial canal of the water-vascular 

 system runs between the two epiphyses in question,* there is no serious 

 obstacle to regarding this pair as corresponding to a pair of ambulacral 

 ossicles. 



The interpretation of the rotulee offers undoubtedly great difficulties. 

 These are not only odd elements as conclusively shown by their origin but 

 also the sole unpaired elements in the lantern of Aristotle. The only odd 

 ossicle which gets into touch with the mouth-frame in extinct star-fishes is 

 the odontophor. In an account of the family Urasterellidse, Spencer (1913, 

 p. 135) observes as follows : — " In a private letter to me .... Hudson asks 

 me to note (in the mouth-frame of the American species Urasterella pulchella)f 

 'paired muscle remains .... just within the large inter-radial (odontophor).' 

 He goes on to state that there must have been muscles to draw in the mouth- 

 angle plates, which in each inter-radius acted as an outer jaw. The origin of 

 these adductors may have been on the inner surface of the odontophors rather 

 than on the first ambulacralia." This indicates that in certain asteroids the 

 odontophor may get into close relationship with the mouth-angle plates. It 

 must be noted, however, that in the lantern of Aristotle the rotula has 

 muscular connection with the epiphyses and not with the jaws. 



In certain other fossil star-fishes again, the odontophor has been shown to 

 occupy an internal position (Spencer, 1917, p. 180). I quote these two 

 features of the odontophor in the extinct asteroidea in order to show how this 

 odd ossicle behaves in certain cases, it being far from my purpose to suggest 

 thereby any asteroid descent for the echinoids. 



The great impediment to regarding the rotulse as modified odontophors is 

 the fact that, while the latter are inter-radial, the former are radial. Can it 

 be that, after occupying an internal position in the ancestral urchin, the 



* This fact will be elucidated in a paper now in preparation, 

 t This parenthesis is mine. 



