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



carried by the mouth-angle plates in star-fishes. If two such bristles get 

 flattened, assume a cone-shape, and are pushed between the two mouth-angle 

 plates thus becoming partly internal, we have the rudiment of an urchin- 

 tooth. 



There must have been several physiological forces at work in the 

 ■evolution of the urchin-tooth. First of all, the liability to wear and tear of 

 an organ used for browsing purposes could have induced the permanent 

 retention of the activity of the embryonic tooth-germ. The ancestor of the 

 sea-urchin, whoever that might have been, let us suppose, started with the 

 five first cones ; these may well have served the primitive animal as organs 

 of mastication. If the ancestral animal browsed on things like the brown 

 fronds of Laminaria or bored into rocks* — certain sea-urchins are known to 

 do both — the conical teeth would be liable to suffer decay. Under such 

 circumstances, the power of replacing worn-out teeth would have been of 

 immense advantage, and hence the permanent activity in the adult of the 

 five embryonic tooth-germs was probably the primary factor in the evolution 

 of the urchin-tooth. 



But how can one account for the coming into being of a stout rod as the 

 result of fusion of several pairs of lamella? ? What could have induced the 

 deposition of numerous pairs of lamella?, one above the other, in the imago 

 urchin, while even as yet the mouth is not formed? Efficiency is the first 

 answer that suggests itself. A short conical tooth, formed by a pair of 

 delicate lamella? and renewed frequently, even at its best must have been 

 but a weak instrument to the ancestral urchin. If the tooth-germs laid 

 down precociously pairs of lamella?, and if by the fusion of these a stout rod 

 resulted, that meant efficiency in function and advance in structure. In this 

 way, one may account for the formation of a stout rod-shaped tooth through 

 the fusion of several cones. 



In this connection, perhaps, it will be well to consider what reaction the 

 evolution of the tooth may have had on the jaws. Each tooth in the adult is 

 closely and immovably attached to a jaw. This being so, the jaw is bound to 

 respond to any adaptations of the tooth. In the " echinus-rudiment," the 

 vertical height of the lantern is at its minimum.f Now, if the tooth 

 increases in length in accordance with the causes indicated, it is extremely 

 likely that there will be a corresponding response on the part of the jaw to 

 adjust itself to the growing tooth. That the tooth may well have been the 



* Vide "The Locomotor Function of the Lantern in Echinus," by Prof. Gemmill, 

 'Eoy. Soc. Proc.,' vol. 85, p. 101 (1912). 



t The height of the lantern in Palceodiscus fero.c is small when compared with that 

 of living urchins (Spencer, 1904). 



